Change boolean callbacks to either complete or error, no values sent

In the case of a callback who's boolean indicates failure but yet no error was
supplied, a generic error is sent to the subscriber. I don't actually know if
this would ever happen, this is just covering the combinational bases.
Introduce default error messages for the account management API

Parse's sample app AnyWall has code to handle these cases, so I'm building it
into this library for convenience.

See: https://github.com/ParsePlatform/AnyWall/blob/b961ceb8ea514b91ff5dea075b142545af5c21b8/AnyWall/PAWLoginViewController.m#L189
Remove unnecessary weakify/strongify