SECURITY: imagemagick/graphicsmagick was ignoring all errors

So, in a much older patch I had a problem where an incorrect RGB
profile would make image uploads fail. I fixed this by using strpos
against the error message...but didn't check the return value
correctly.

That means that any error from gm/im was ignored. This caused people
to upload too large images and flood /b/ with 1 x 10000 pixel images

My fault, patched now. Sorry about that.
Merge https://github.com/ctrlcctrlv/8chan