wand.exceptions.WandError gets overwritten

[mcirello]

In exceptions.py you have an entry in the domain map:

 ('Wand', 'There was a problem specific to the MagickWand API.', (),
     [370, 470, 770]),

This creates a "WandError" that overwrites the base class WandError. So this doesn't work:

import wand.exceptions

try:
  raise wand.exceptions.CoderError
except wand.exceptions.WandError:
 pass

[mlissner]

Just got bit by this too and wasted some time trying to figure out why I couldn't catch an exception.

If the fix is in and we can get the memory fixed in #300, can we do a release?