Improve API Error Messages

[ab-10]

Currently for both API endpoints (/properties/api and /roleBasedVote/api) a request that does not have all of the required parameters returns {Invalid request}.

There are two problems with this:

1. The response is not informative enough. The response should specify which parameters were missing.
2. For property handler, it should be permitted to have a missing comparison if the properties is set to p and missing pivot if properties is set to c.