Improve error formatting

Changes:

* The divider lines are gone. These were in practice a bit confusing,
  in particular with --show-trace or --keep-going, since then there
  were multiple lines, suggesting a start/end which wasn't the case.

* Instead, multi-line error messages are now indented to align with
  the prefix (e.g. "error: ").

* The 'description' field is gone since we weren't really using it.

* 'hint' is renamed to 'msg' since it really wasn't a hint.

* The error is now printed *before* the location info.

* The 'name' field is no longer printed since most of the time it
  wasn't very useful since it was just the name of the exception (like
  EvalError). Ideally in the future this would be a unique, easily
  googleable error ID (like rustc).

* "trace:" is now just "…". This assumes error contexts start with
  something like "while doing X".

Example before:

  error: --- AssertionError ---------------------------------------------------------------------------------------- nix
  at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

       6|
       7|   x = assert false; 1;
        |       ^
       8|

  assertion 'false' failed
  ----------------------------------------------------- show-trace -----------------------------------------------------
  trace: while evaluating the attribute 'x' of the derivation 'hello-2.10'
  at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

     191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
     192|           name = "${attrs.pname}-${attrs.version}";
        |           ^
     193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

Example after:

  error: assertion 'false' failed

         at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

              6|
              7|   x = assert false; 1;
               |       ^
              8|

         … while evaluating the attribute 'x' of the derivation 'hello-2.10'

         at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

            191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
            192|           name = "${attrs.pname}-${attrs.version}";
               |           ^
            193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

src/libexpr/eval.cc

@@ -622,7 +622,7 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2))
 {
     throw EvalError({
-        .hint = hintfmt(s, s2),
+        .msg = hintfmt(s, s2),
         .errPos = pos
     });
 }
@@ -635,7 +635,7 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, con
 LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2, const string & s3))
 {
     throw EvalError({
-        .hint = hintfmt(s, s2, s3),
+        .msg = hintfmt(s, s2, s3),
         .errPos = pos
     });
 }
@@ -644,7 +644,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & p1, const char * s, const
 {
     // p1 is where the error occurred; p2 is a position mentioned in the message.
     throw EvalError({
-        .hint = hintfmt(s, sym, p2),
+        .msg = hintfmt(s, sym, p2),
         .errPos = p1
     });
 }
@@ -652,7 +652,7 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & p1, const char * s, const
 LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 {
     throw TypeError({
-        .hint = hintfmt(s),
+        .msg = hintfmt(s),
         .errPos = pos
     });
 }
@@ -660,7 +660,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const ExprLambda & fun, const Symbol & s2))
 {
     throw TypeError({
-        .hint = hintfmt(s, fun.showNamePos(), s2),
+        .msg = hintfmt(s, fun.showNamePos(), s2),
         .errPos = pos
     });
 }
@@ -668,7 +668,7 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s, const string & s1))
 {
     throw AssertionError({
-        .hint = hintfmt(s, s1),
+        .msg = hintfmt(s, s1),
         .errPos = pos
     });
 }
@@ -676,7 +676,7 @@ LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s,
 LocalNoInlineNoReturn(void throwUndefinedVarError(const Pos & pos, const char * s, const string & s1))
 {
     throw UndefinedVarError({
-        .hint = hintfmt(s, s1),
+        .msg = hintfmt(s, s1),
         .errPos = pos
     });
 }
@@ -684,7 +684,7 @@ LocalNoInlineNoReturn(void throwUndefinedVarError(const Pos & pos, const char *
 LocalNoInlineNoReturn(void throwMissingArgumentError(const Pos & pos, const char * s, const string & s1))
 {
     throw MissingArgumentError({
-        .hint = hintfmt(s, s1),
+        .msg = hintfmt(s, s1),
         .errPos = pos
     });
 }
@@ -2057,7 +2057,7 @@ void EvalState::printStats()
 string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({
-        .hint = hintfmt("cannot coerce %1% to a string", showType()),
+        .msg = hintfmt("cannot coerce %1% to a string", showType()),
         .errPos = pos
     });
 }