Fix namespace error handlers when propagate_exceptions=True

[mjreiss]

Fixes issue #285.

When an errorhandler is registered on a namespace, and PROPAGATE_EXCEPTIONS is set to True in the Flask app, then the namespace handler will not catch the exceptions. It looks like this is due to the handle_error function not checking the error handlers that exist in any child classes.

[codecov]

Codecov Report

Merging #286 (314458e) into master (41b3b59) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #286   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files          19       19           
  Lines        2712     2712           
=======================================
  Hits         2627     2627           
  Misses         85       85           

Impacted Files	Coverage Δ
flask_restx/api.py	96.69% <ø> (ø)

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