Nothing too important, but it helps localizing the cause of an error
much faster.
By the way, I also changed the output for assertThrows and
assertDoesNotThrow a bit.
All new arguments are optional, so everything is backwards compatible.

R=jfb@chromium.org, titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1866693002

Cr-Commit-Position: refs/heads/master@{#35322}