added slightly-magic error handling behavior to KeywordHandler.
added slightly-magic error handling behavior to KeywordHandler.

the most common exceptions to raise from handlers are DoesNotExist and
ValueError (where an int was expected, but something else arrived), so
i'm wrapping those in a helpful error message.

these errors can (and in most cases, should) be caught within the
handler, and given a more specific error message. but this is a nice
default behavior.

this should be refactored into BaseHandler, but currently there's
no sensible place to put it.

rapidsms/rapidsms-contrib-apps-dev,rapidsms/rapidsms-contrib-apps-dev