mailer.py: Fix breakage on Python 2 from use of Python 3 syntax
Follow-up on r1869194. That revision addresses issue SVN-1804 by adding exception handling for SMTP errors. Unfortunately a last-minute change in that revision uses Python 3+ 'raise..from' syntax. The script does not yet support Python 3 so this breaks the script for all Python versions!
* tools/hook-scripts/mailer/mailer.py (SMTPOutput.finish): Comment out the two instances of 'from detail' that break the script, but add a 'TODO' note to uncomment them when converting the script to Python 3.