Catch timeouts in quietly_try_to_purge

Although many times of errors are caught, the case where a connection
is made but no reply is received resulted in an uncaught Timeout::Error.
Use exception notification for third party url errors

Include URL and error in notification and log.
Merge branch 'catch-openuri-timeout-errors' into rails-3-develop
Merge branch 'rails-3-develop' of ssh://git.mysociety.org/data/git/public/alaveteli into rails-3-develop