Add error messages before returning NULL when encoding

[radarhere]

While I'm here, I've also removed

Pillow/src/encode.c

Lines 1346 to 1347 in 9a89944

	} else if (strcmp(format, "jpt") == 0) {
	codec_format = OPJ_CODEC_JPT;

since "jpt" is never used.

Pillow/src/PIL/Jpeg2KImagePlugin.py

Lines 382 to 385 in 9a89944

	if filename.endswith(b".j2k") or info.get("no_jp2", False):
	kind = "j2k"
	else:
	kind = "jp2"