fix(vcs): sanitize mercurial error messages

[nijel]

It includes lengthy detail on the failed HTTP response here what is not really useful to show in the UI.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.57%. Comparing base (3b92090) to head (38acdb8).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/vcs/mercurial.py	66.66%	1 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (80.00%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

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