[Libguestfs] [PATCH libnbd v2] python: Raise a custom exception containing error string and errno.

This kind of fixes the problems in v1. The exception still primarily lives in the libnbdmod and you could still refer to it using libnbdmod.Error (but don't do that). However when the exception is printed it now appears as nbd.Error, and you can catch it also using the same name. Other problems: - There is no "nice" interface to accessing the exception fields. You have to use ex.args[0] and ex.args[1]. - The errno is represented as a number, which means it will be hard to write portable Python code depending on the errno. Rich.