Throw ImportError on shared library load failure #17
If the MagickWand library isn't installed, wand triggers an AttributeError when it gets to this line:
library.NewMagickWand.restype = ctypes.c_void_p
This exception falls through all the way to the orignal import of wand, e.g.:
This pull request catches the AttributeError and raises an ImportError instead, so this can be cleanly handled by the module that imports wand.