If an invalid shader is generated, generate the appropriate message
instead of "Unknown error".

Bug: angleproject:3394
Change-Id: I586c99e95cd72daeaf74b4d1eff352942e01d8b7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1982783Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>