This is a small tweak to the logic added in cl/1511 to detect explicit
throws when printing stack traces. Now when deciding whether to print
"caused by explicit throw", it checks not only that the error is a
ThrowError and that we are in a throw, but also that the ThrowError was
thrown by *this* throw, and not by another while in the process of
evaluating this throw's operand.
It turns this:
```
let
set = {
inner = throw "nested throw";
};
in
throw set.inner
error:
… caused by explicit throw
at /pwd/in.nix:6:1:
5| in
6| throw set.inner
| ^
7|
… while evaluating the attribute 'inner'
at /pwd/in.nix:3:5:
2| set = {
3| inner = throw "nested throw";
| ^
4| };
… caused by explicit throw
at /pwd/in.nix:3:13:
2| set = {
3| inner = throw "nested throw";
| ^
4| };
error: nested throw
```
into this:
```
error:
… while calling the 'throw' builtin
at /pwd/in.nix:6:1:
5| in
6| throw set.inner
| ^
7|
… while evaluating the attribute 'inner'
at /pwd/in.nix:3:5:
2| set = {
3| inner = throw "nested throw";
| ^
4| };
… caused by explicit throw
at /pwd/in.nix:3:13:
2| set = {
3| inner = throw "nested throw";
| ^
4| };
error: nested throw
```
Notice the difference in the top context frame. Before it incorrectly
attributed the throw error to the both throws instead of just the one
that actually threw.
Change-Id: If3b0b8311f1ae2ff1471e260fe59d9166a6a6964
skye
e79278b4fc