Right now, all these exceptions are returned in HTML. For example,

http post http://0.0.0.0:5005/api/v1.0/decision decision_context="bodhi_update_push_stable" Accept:'application/json' product_version="fedora-27" subject:='["glibc-1.0-1.f26"]'

HTTP/1.0 404 NOT FOUND
Content-Length: 161
Content-Type: text/html
Date: Thu, 20 Jul 2017 05:31:39 GMT
Server: Werkzeug/0.12.2 Python/2.7.13

Not Found

Cannot find any applicable policies for fedora-27

It would be good to return JSON to make other apps quering Greenwave life easier. We probably just need to add some errorhandlers in Flask.