Make better error message from validation errors

[wbobeirne]

Right now validation 400 errors get sent back from the api as:

{ "message": "Something went wrong" }

However, the error actually has some rich data to it, e.g.

{'comment': ['Longer than maximum length 1000.']}

It would be nice to present an error that uses this message instead of a generic one.

[dternyak]

A few links that point to code related: https://github.com/ZcashFoundation/zcash-grant-system/blob/develop/backend/grant/app.py#L42-L80

In particular, we'll want to ensure that comments longer than 1000 characters have better error responses: https://github.com/ZcashFoundation/zcash-grant-system/blob/develop/backend/grant/proposal/views.py#L101-L104