Fix, improve and test error traces

[layus]

Motivation

I did some heavy lifting work on the traces output some time (years?) ago, but in the meantime some of the design principles I tried to apply were reverted.

This time, I reintroduce the opinionated changes, together with a proper testing framework and tests.

Context

We already display the trace from outer to inner, in order to have the innermost trace closer to the actual error message, at the bottom of the terminal, supposedly closer to where the user lands after the error.

At the time I ensured that the innermost error traces were kept while truncating, but it got somehow reversed, and now only the outermost ones are displayed.
So I returned to display innermost ones, add tests, and moved the truncation warning at the top, where the truncation actually happens.

I have also take the opportunity to add a better error message at the top, saying what we were doing while erroring. Hopefully this can help to know which attribute failed in situation where we build multiple installables for example.

$ nix eval -f /tmp/complex.nix
error: during evaluation of file '/tmp/complex.nix'

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)
       
       … while evaluating the attribute 'value'
         at /tmp/complex.nix:20:18:
           19|   # Now do complex operations with nested calls
           20|   helper1 = x: { value = x; };
             |                  ^
           21|   helper2 = x: (helper1 x).value + 1;

       … while evaluating the attribute 'value'
         at /tmp/complex.nix:20:18:

       … while calling the 'throw' builtin
         at /tmp/complex.nix:27:20:
           26|   # This will fail deep in the evaluation
           27|   final = helper5 (throw "computation error deep in the stack");
             |                    ^
           28| in

       error: computation error deep in the stack

Now, I am not too sure about the tests locations, but otherwise I think it is a much needed improvement test-wise.

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[xokdvium]

This seems quite bad, we'd rather avoid more globals. This can be achieved by just inserting a catch block in the relevant code? Also seems quite similar to what traces at the top (outermost) can include additionally?

[xokdvium]

Overall the fix with reversing the order is great, thanks! @roberth tells me that this got mangled by a concurrent PR back when the reversal was done?

I think in general we aligned on having more characterisation tests in the lang test suite for error traces, as that's much more maintainable then handwritten unit tests that also hardcode the ANSI escapes. The coverage there is pretty good overall, and maybe a good starting point to land this would be to add a bad test, which gets fixed in a subsequent commit/PR?

[rhendric]

(Comment in a somewhat arbitrary place to allow for threading.)

Suggestion from the peanut gallery, since you're working on how traces are filtered and presented: how about always including the while evaluating derivation '%s' trace from src/libexpr/primops.cc, as a way of implementing #10061? If this is too unrelated, I could offer it in a separate PR, but I didn't want to step on your toes here.