Default error serializer content negotiation

[copalco]

Summary of Changes

We would like to use choose from available content type handlers to return error response content in a best format basing on the accept header of the request.

Related Issues

#2023

Pull Request Checklist

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[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (52db96d) 100.00% compared to head (0e4b6f8) 36.78%.

Files	Patch %	Lines
falcon/app_helpers.py	20.00%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##            master    #2190       +/-   ##
============================================
- Coverage   100.00%   36.78%   -63.22%     
============================================
  Files           62       62               
  Lines         6871     6878        +7     
  Branches      1098     1098               
============================================
- Hits          6871     2530     -4341     
- Misses           0     4294     +4294     
- Partials         0       54       +54     

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