trying to fix query 500 errors
Caused ESView.run_query to raise an exception on bad user input
I also made a new exception "ESUserError" for this purpose.  Subclasses
should catch this exception manually.
Added BadRequestError exception to reports, and handling to return a 400 in the case of that error
Merge branch 'master' of https://github.com/dimagi/commcare-hq into user_query_fix
added separate error message for bad querystring.  Feel free to change
Merge pull request #795 from esoergel/user_query_fix

Fix for bad user queries in reports