Improved error log for invalid keys

- `pressed()` in FlxInputStates now also calls `FlxG.log()`. Otherwise, you're left clueless why the key code you're trying to use doesn't work
- The Key string is now logged as well, making the error log more
helpful.
- The error log also covers the possiblity of the key being invalid
without being a function or number key. Before, it just assumed that the invalid key string is caused by the use of numpad or function keys.

#318