Throw exceptions instead of returning falsy values

Key module defines exceptions since Add Key-specific exceptions → but it does not use them anywhere. All methods work like procedural codes usually, returns NULL or FALSE if an error occurs. There is no method description that would define that any of the described exceptions could be thrown by the method.

Suggested solution:
Instead of returning falsy values, throw appropriate exceptions with useful exceptions messages and exceptions codes. This is probably a BC breaking change so it could be implemented only in new 7.x-4.x and 8.x-2.x branches.