R1:99b9ceb6d774

Fix arcanist documentation to include "set -e" in example SVN hooks

Summary: "set -e" causes the script to exit if any command returns nonzero.
Without it, we incorrectly discard the failure code. At Facebook everything runs
in "set -e" or some equivalent so I never picked this up in testing.

Test Plan:
Added the hook to my svn local, it blocked bad commits and allowed good ones.

>>> orbital:~/devtools/svnroot $ svn commit -m "quaa"
Sending test.php
Transmitting file data .svn: Commit…