Translate encoder error codes to strings; deprecate ImageFile.raise_oserror()

[bgilbert]

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.

Replace all callers of raise_oserror() with raise _get_oserror(). Deprecate raise_oserror(), since it doesn't need to be in the public API.

Split out from #7553 (comment).

[bgilbert]

Should we switch all the internal callers to _get_oserror() and deprecate raise_oserror() entirely? It seems like the functionality is only useful for code that calls into libImaging, i.e. not for external callers. Its predecessor raise_ioerror() was inherited from PIL as an exported function.

Alternatively, if we don't want to do that, should we expose the new encoder argument on raise_oserror()? It's a bit awkward that raise_oserror() will have less functionality than the helper function it wraps.

[radarhere]

Alternatively, if we don't want to do that, should we expose the new encoder argument on raise_oserror()? It's a bit awkward that raise_oserror() will have less functionality than the helper function it wraps.

That wouldn't completely simplify things though, would it? Because it is still ideal to raise the error from another in the following code

raise _get_oserror(errcode, encoder=True) from exc

which raise_oserror() can't do, with or without an encoder argument, because doesn't return an error, it immediately raises it.

While Pillow values backwards compatibility, I agree that raise_oserror() doesn't appear useful to users, so deprecating it sounds fine to me.

[bgilbert]

Sounds good, done.

[bgilbert]

Added @radarhere's changes from bgilbert#4.