It is valid for the implementation to return a
GL_FRAMEBUFFER_UNSUPPORTED for framebuffer completeness. Check for that
case and return early. This fixes the RGB5_A1 tests on Mac that doesn't
support rendering to this format.

BUG=angleproject:891

Change-Id: If37d8f82e994a93a470cbdc01d758fe60d54a5b2
Reviewed-on: https://chromium-review.googlesource.com/298863Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>