Added the Exception part of the Uaapi.

This define 3 classes:

* Uaapi::Exception, used as the base class. It takes a string at
  construction corresponding to the exception message. It has a what()
  method that will return this error string.
* Uaapi::FatalError, used for errors that should terminate execution.
* Uaapi::RuntimeError, used for errors that can and should be
  recovered from.
Added missing include in Exception.h.