Raise error if ImageGrab subprocess gives non-zero returncode

[radarhere]

#9070 (comment) suggested checking the subprocess return code in ImageGrab. This PR adds it, following the example of

Pillow/src/PIL/GifImagePlugin.py

Lines 909 to 910 in b0a5bc2

	if retcode:
	raise subprocess.CalledProcessError(retcode, quant_cmd)

I've also added additional testing, as per #9070 (comment), including opening a new window on macOS to test capturing it. To avoid interfering too much with user machines though, this will only happen on CIs.