UNSUPPORTED is a specific error meaning an implementation-specific
incompatibility. For cases where the attachments are invalid according
to the spec, we should be returning INCOMPLETE_ATTACHMENT.

BUG=angleproject:1231

Change-Id: I4b4332ee5661738374e93c825a53d66d7ba1fad5
Reviewed-on: https://chromium-review.googlesource.com/313995
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>