Detect mercurial errors

Start by tracing the command execution path that produces the shown stderr from the issue body. Check how stderr is currently captured and logged; done means traceback-containing Mercurial stderr produces a Sentry exception without aborting the command.

It appears that the error from Bug 1916544 actually wasn't breaking things; however it was spitting out this error into stderr and not causing any warnings/error from Updatebot (presumably because the command was not erroring):

[Debug]  (CommandProvider xsimd job_id=549) ----------------------------------------------
[task 2024-09-02T18:11:54.074Z] [Info]   (CommandProvider xsimd job_id=549) Running ['hg', 'checkout', '-C', '.']
[task 2024-09-02T18:11:56.845Z] [Info]   (CommandProvider xsimd job_id=549) Return: 0 Runtime (s): 2
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) -------
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) stdout:
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) 0 files updated, 0 files merged, 0 files removed, 0 files unresolved
[task 2024-09-02T18:11:56.845Z] 
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) -------
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) stderr:
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) Traceback (most recent call last):
[task 2024-09-02T18:11:56.845Z]   File "/usr/lib/python3/dist-packages/mercurial/extensions.py", line 255, in _runuisetup
[task 2024-09-02T18:11:56.845Z]     uisetup(ui)
[task 2024-09-02T18:11:56.845Z]   File "/builds/worker/.mozbuild/evolve/hgext3rd/evolve/exthelper.py", line 149, in finaluisetup
[task 2024-09-02T18:11:56.845Z]     c(ui)
[task 2024-09-02T18:11:56.845Z]   File "/builds/worker/.mozbuild/evolve/hgext3rd/evolve/headchecking.py", line 28, in uisetup
[task 2024-09-02T18:11:56.845Z]     code = scmutil.filteredhash.__code__
[task 2024-09-02T18:11:56.845Z]            ^^^^^^^^^^^^^^^^^^^^
[task 2024-09-02T18:11:56.845Z] AttributeError: module 'mercurial.scmutil' has no attribute 'filteredhash'
[task 2024-09-02T18:11:56.845Z] *** failed to set up extension evolve: module 'mercurial.scmutil' has no attribute 'filteredhash'
[task 2024-09-02T18:11:56.845Z] 
[task 2024-09-02T18:11:56.845Z] [Debug]  (CommandProvider xsimd job_id=549) ----------------------------------------------

We should automatically look at stderr output and detect if there is a traceback in it, and if so, log an exception to sentry (but not abort execution).