[Libguestfs] [PATCH libnbd v3] python: Raise a custom exception containing error string and errno.

[Richard W.M. Jones]

Following Eric's suggestions from v2, this adds .string, .errno and
.__str__ properties.  The .string property returns the error string.
The .errno property returns the errno (from the errno module), or
None.  The __str__ property makes the exception nicely printable.

Rich.