Validation errors return status code 400 where as it should be 422

[karthikkp]

No description provided.

[Zertz]

Good point. I can probably roll out this change in #358

[karthikkp]

I just saw that even CastError returns 400. Basically it would be better if all mongoose related errors are put in 422 and if a field name is spelt wrong, or if the "id" specified is wrong a 400 will be better. So on the front end, all 422 errors, we can display the error message that comes with the response(examples - email already exists, passwords don't match,validation errors), whereas for 400 "the fault is with the client that sends the request" and we can display a generic error message like "something went wrong". So 400 errors need to be fixed on the frontend and 422 errors don't need fixing.