Option to conditionally put exception traceback into response

If error response was caused by an exception (including call
to flask.abort()), it is possible to add traceback information to
exception to response in order to make debugging easier and
bug reports more helpful. With this commit we provide an option
to configure this behavior.

Change-Id: Icdfe3e35d2f4a28d76a727193db2738c1d642224