Better handle errors display as JSON or XML

[stanislas-m]

• Make sure using the XML mime type returns a XML-formatted error.
• Use the same struct for XML & JSON.
• Add a new attribute to have only the cause and not the full trace.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (development@1cd0d04). Click here to learn what that means.
The diff coverage is 73.33%.

@@              Coverage Diff               @@
##             development    #1351   +/-   ##
==============================================
  Coverage               ?   48.82%           
==============================================
  Files                  ?       92           
  Lines                  ?     4057           
  Branches               ?        0           
==============================================
  Hits                   ?     1981           
  Misses                 ?     1969           
  Partials               ?      107

Impacted Files	Coverage Δ
errors.go	75% <73.33%> (ø)

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[stanislas-m]

Fixes #1353.