Give more descriptive error messages.
Give more descriptive error messages.

Summary:
Now each error is a dict containing the param name, helpstring, and
error message as separate params, rather than trying to combine them
into one (confusing) string.

Test Plan:
Fixed webapp tests to test the new error format, and ran
   tools/runtests.py api/*_test.py route_util/

Reviewers: alan

Subscribers: davidwang

Differential Revision: https://phabricator.khanacademy.org/D21579

Khan/flask-restful