- One regression introduced by the "AM errors as object" features is
  about the `full_messages` method.

  It's currently impossible to call that method if the `base` object
  passed in the constructor of `AM::Errors` doesn't respond to the
  `errors` method.
  That's because `full_messages` now makes a weird back and forth trip

  `AM::Errors#full_messages` -> `AM::Error#full_message` -> `AM::Errors#full_message`

  Since `full_message` (singular) isn't needed by AM::Errors, I moved
  it to the `AM::Error` (singular) class. This way we don't need to
  grab the `AM::Errors` object from the base.