Allow exception propagation when app.propagate_exceptions is set

[dtheodor]

Added a check in Api.handle_error that propagates exceptions when Flask.propagate_exceptions is set. This honors the app.testing setting as established at http://flask.pocoo.org/docs/api/#flask.Flask.test_client that propagates exceptions instead of returning Flask responses. Otherwise behavior is inconsistent with Flask and testing is harder.

In general it would be nice to use Flask's error handlers instead of hacking a level above them, this would alleviate having to place another check (propagate_exceptions is already checked in flask). I am not sure if the handlers are
good enough to support wrapping all exceptions into json responses though.

[dougblack]

Thanks for this.

Flask-RESTful provides it's own error handlers so users that have a Flask-RESTful routes living beside regular Flask routes can handle them based on the type of route.