Explictly check if the internal format enum is ever valid before checking
if it is valid in combination with other parameters. Some WebGL tests
expect a certain order for generated errors.

BUG=angleproject:2009
TEST=conformance2/textures/misc/tex-input-validation

Change-Id: I31166a78d00629f8281ef53eced72575497ae448
Reviewed-on: https://chromium-review.googlesource.com/486099
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>