This CL revises some of our error messages, and removes unneeded parts
(like "AsyncCompilation: " or "(null): "). It also extends existing
tests to check for the precise error message more thoroughly to detect
changes or nondeterminism earlier.

R=titzer@chromium.org, ahaas@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Bug: chromium:926311
Change-Id: I1ccfb307d4a61291f4582330152a53fbadd0848f
Reviewed-on: https://chromium-review.googlesource.com/c/1445897
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59296}