It's only useful if the caller has an IMAGING_CODEC_* error code, which
are only produced by codec decode() methods and are automatically
translated by ImageFile.
Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>
When decoding, we use raise_oserror() to convert codec error codes to
strings. Adapt that code to be used when encoding as well. Add a new
helper function that returns the exception so we can still raise
`from exc`.
