500 -> better error message if location view breaks

http://manage.dimagi.com/default.asp?149400#838049
Looking at the stack trace in the ticket - I think it's a little dangerous to override `.get` here and generally it's a good rule to trap exceptions as close to the code you expect to call them.

What I think would be a bit cleaner is if in the `supply_point` function you catch the `MultipleResultsFound` error and then reraise a `BadCommTrackConfigException` or something. I'd also recommend trapping that error in the context function instead of the `get` method and ideally just providing a default (e.g. empty) consumption value if that's possible.