Fix `mb validate`, which was broken by the args-file support I added.

Also, delete a couple of bot entries for bots that were shut down,
and make sure we actually print the stack trace to be helpful when
we raise errors.

[email protected]
BUG=481692

Review URL: https://codereview.chromium.org/1892403002

Cr-Commit-Position: refs/heads/master@{#388151}
1 file changed