ClutterTexture relies too much on GError, even for things that are
clearly programmer errors. Also, no error message passed to GError
is marked for translation as it should.

We should move the programmer errors, like passing the wrong bpp
value with regards to the presence of the alpha channel, to real
warnings; we should also try and harmonize all the error messages,
and not mention Cogl — especially in the ones marked for translation.