libexpr: fix --debugger --ignore-try

a65e9e5828 did not inform `tryEval` that
(as far as it's concerned) `state.debug` moved to `state.errors.debug`
and changed types. this resulted in the REPL erroneously coming up, that
REPL having a non-debug state, and segfaulting after that REPL exited.

it's probably good that `state.debug` isn't mutated by `--ignore-try`
anymore.

Change-Id: I1918e93edacd626452aa423fc2eb825080738835
Fixes: a65e9e5828 ("libexpr: extract eval error creation into new type")
Signed-off-by: Dusk Banks <me@bb010g.com>
(cherry picked from commit 6a583136b7)

tests/functional/repl_characterization/data/debug_ignore_try.test

@@ -0,0 +1,8 @@
+@args --debugger --ignore-try
+
+we don't enter a debug repl through tryEval
+  nix-repl> (builtins.tryEval ((x: throw "foo") 1)).success
+  false
+
+no segfault either
+  nix-repl> :quit

tests/functional/repl_characterization/repl_characterization.cc

@@ -187,5 +187,6 @@ REPL_TEST(stack_vars);
 REPL_TEST(errors);
 REPL_TEST(idempotent);
 REPL_TEST(debug_frames);
+REPL_TEST(debug_ignore_try);
 
 }; // namespace nix