Change the exception type when parsing bad data

[jleclanche]

Right now PIL/Pillow recommends raising SyntaxError when dealing with bad data. Example in PaletteFile:

            if len(s) > 100:
                raise SyntaxError("bad palette file")

It'd be a lot nicer raising a custom exception. Abusing SyntaxError is nasty.

[radarhere]

Hi. Thanks for contributing with this suggestion. My only input here is that there are many SyntaxError exceptions thrown throughout the image plugins, so if done properly, this is actually a bigger change than simply PaletteFile.

[jleclanche]

I agree, yes.

[meawoppl]

Each encoder should catch and homogenize potential errors thrown to this more canonical form.

[aclark4life]

@jleclanche You want to take a whack at this one?

[jleclanche]

@aclark4life yeah sure, but this is quite the backwards incompatible change. fine with that?

[wiredfool]

If only we had looked at a list of things to do for a breaking change release

[radarhere]

#7425 and #7489 would also like custom exceptions instead of OSErrors.