PySVN and Subvertpy will raise exceptions when there are communication

errors fetching the logs for a path in Subversion. These end up causing

error e-mails or log file entries, and result in generic "HTTP 500" error

messages reported on the New Review Request page.
This change adds exception handling that looks for any exceptions from

the SVN backend and converts them to SCMErrors. These will display

nicely in the New Review Request page.

Unit tests passed.
Tested that the New Review Request page properly showed errors bubbling

up from the backend.