Arcyd: fix dashboard reporting no users on branch

We should report there being no users on a branch regardless of whether
we're mailing on failure.

Test Plan:
$ ./testbed/arcyd/test_shell.sh
manually poke a review with an invalid author
observe that the error is now reported in full on the dashboard
$ ./precommit
Arcyd: add more detail to 'no users' error

Add more detail to the 'Unable to assign any users to branch.' error
message that is displayed on the dashboard, better to suggest some
things for people to copy-paste then just refer them to the docs.

Test Plan:
$ ./testbed/arcyd/test_shell.sh
manually create a review with bad author, it turns out that '<text>'
won't appear on the dashboard as it's getting converted to a tag, will
deal with that separately.

$ ./precommit.sh
Arcyd: html escape text before adding to <pre>'s

To prevent accidentally injecting html tags into free text, html escape
the text.

E.g. '$ git config --global user.email <VALID EMAIL>' would
inadvertantly create a '<VALID EMAIL>' tag unless escaped

Test Plan:
$ ./testbed/arcyd/test_shell.sh
Replace the 'bad author' message with something that uses chevrons and
manually push a bad author review branch and inspect the results.
$ ./precommit.sh