"set -e" was aborting the script after grep doesn't find the Bug line,
and "set -x" makes things overly verbose. So perform the validation
before these options are set.

Also return 1 instead of 0 on failure.

Bug: b/141892461
Change-Id: Ica0364ff49383b47cf37b3a02d6b03e55d8d88ad
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37008
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>