Better error/Exception handling

[CodeProKid]

There's a few places in the DFM_Transients class that we throw Exceptions for things like trying to retrieve data for a transient that hasn't been registered through the registration system. We should either handle these exceptions and do something with them, or just make them WP_Error objects.

The way things are currently setup cause an IDE to light up like a Christmas tree every time you use the API since there are unhandled exceptions in the underlying code.