exception types could use a common base class

[foresto]

The exception classes (as inherited from dbus-next) derive directly from various python exceptions, rather than sharing a single superclass.

Because of this, distinguishing dbus-related errors from other errors requires application code to wrap dbus-fast calls in their own try/except blocks, and wrap other calls in separate try/except blocks (or else catch every dbus-fast exception individually by name). The result is very cluttered code.

A coherent exception hierarchy would make dbus-fast much nicer to work with, and allow application code to be simpler and easier to read.

[bdraco]

Feel free to open a PR to clean this up