Do not raise error from unknown channel id when parsing PSD layers

[radarhere]

Within a PSD layer, it is possible that an unsupported channel ID will trigger an IndexError.

Pillow/src/PIL/PsdImagePlugin.py

Lines 224 to 232 in ee9e761

	for _ in range(ct_types):
	type = i16(read(2))
	if type == 65535:
	b = "A"
	else:
	b = "RGBA"[type]
	bands.append(b)

However, Pillow has already established that if the combination of channels don't lead to a recognised mode, that's not cause for an error.

Pillow/src/PIL/PsdImagePlugin.py

Lines 235 to 244 in ee9e761

	# figure out the image mode
	bands.sort()
	if bands == ["R"]:
	mode = "L"
	elif bands == ["B", "G", "R"]:
	mode = "RGB"
	elif bands == ["A", "B", "G", "R"]:
	mode = "RGBA"
	else:
	mode = "" # unknown

This PR applies that same logic to the first step, since conceptually, not knowing what type of data is in a band is a subset of not knowing what mode the overall layer is.