Clarified error message

[radarhere]

Pillow/src/decode.c

Lines 298 to 301 in 2563727

	unpack = ImagingFindUnpacker(mode, rawmode, &bits);
	if (!unpack) {
	Py_DECREF(decoder);
	PyErr_SetString(PyExc_ValueError, "unknown raw mode");

#4818 (comment) suggests that "unknown raw mode" is not completely descriptive here. This is a fair comment - it's not that the raw mode is unknown, it's that our list of unpackers doesn't have a mapping from the raw mode to the mode. The particular raw mode may be valid for other modes.

So this PR changes the text, mostly using the suggestion from the comment.