Upgrade vendor to get newest raven (sentry client).
Improve error reporting to Sentry.

We had some duplicate errors going due to
logging when raising exceptions and when
catching them. Exception messages were also
too unique and thus weren't being grouped
properly.
Merge pull request #89 from pmclanahan/improve-error-reporting

Improve error reporting to Sentry.
Update subscribe docs to include source_url
Bug 912055: Validate email addresses.

Also suggests spelling corrections when available.