Old code had a mix of framebuffer error codes from OpenGL ES, EXT_framebuffer_object, and desktop GL. We use desktop GL (or ARB_framebuffer_object which acts just like GL 3.x) so I made it compatible with that.
Changed messages to the actual GL_FRAMEBUFFER_XXX symbols. These are less friendly and more accurate. Can easily look up what an error means, unfiltered by what a Blender dev thinks it means.
Kept ES error codes around in case we support that one day. Just flip the #if or use a compile-time option.
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