This CL revises some of our error messages to be more precise or more
aesthetically appealing.

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

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Bug: chromium:926311
Change-Id: I38eaee09fd37f9b67fdb08bc7b0df64a6eaf96f9
Reviewed-on: https://chromium-review.googlesource.com/c/1445980Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59249}