Implement /__error__ endpoint

[leplatrem]

See mozilla-services/Dockerflow#75
And https://mozilla-hub.atlassian.net/browse/RMST-332

[codecov-commenter]

Codecov Report

❌ Patch coverage is 21.42857% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.88%. Comparing base (9014f6e) to head (f9cb8cd).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dockerflow/fastapi/views.py	0.00%	5 Missing ⚠️
src/dockerflow/django/views.py	33.33%	2 Missing ⚠️
src/dockerflow/flask/app.py	33.33%	2 Missing ⚠️
src/dockerflow/sanic/app.py	33.33%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (9014f6e) and HEAD (f9cb8cd). Click for more details.

HEAD has 45 uploads less than BASE

Flag	BASE (9014f6e)	HEAD (f9cb8cd)
	54	9

Additional details and impacted files

@@ Coverage Diff @@## main #123 +/- ##
===========================================
- Coverage 97.84% 85.88% -11.97% 
===========================================
Files 22 22 Lines 744 758 +14 Branches 97 73 -24 ===========================================
- Hits 728 651 -77 - Misses 9 104 +95 + Partials 7 3 -4 

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