Friendlier errors on post creation

[joshsmith]

Post creation without a title gives you something like:

The adapter rejected the commit because it was invalid

Ideally we should try to give the user a better error.

[begedin]

That's a general thing with non-validation adapter errors. From what I can tell, the default behavior of the JSON API adapter is the replace those errors with this generic one. I'll have to override a method in the application adapter, but we definitely should do this.

That being said, this should be a validation error, so there must be a problem there as well.