[Image-SIG] more descriptive error for image.paste

[Janto Dreijer]

I'd really appreciate it if Image.paste can be more explicit about
what went wrong than just "ValueError: images do not match".

I've been sitting here for at least an hour trying to figure out why
the images are incompatible. Different color mode? Pasted image size
!= bbox size? bbox outside image? Wrong parmeters? Argh! Just tell me!

Thanks
Janto