Return helpful errors

[matthewmueller]

Whenever a request doesn't match the controller's signature, we should return a 400 Bad Request with a helpful error:

For example, given the following signature:

func (c *Controller) Create(name, email string) {}

With the following request map:

{
  "name": 123
}

We should return the following response:

{
  "error": "name must be a string and email can't be blank",
  "fields": {
    "name": "must be a string",
    "email": "can't be blank"
  }
}

Errors should also be helpful for nested inputs:

{
  "error": "version can't be blank",
  "fields": {
    "op.params[1].version": "can't be blank"
  }
}