This helper returns the appropriate error with the user specifies
an attachment that is invalid or out-of-range (for indexed color
attachments).

BUG=angle:571

Change-Id: I80ed347e3540579110e40e742fbacb0467cb85fd
Reviewed-on: https://chromium-review.googlesource.com/205604Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>