Check for valid newsletters before creating subscriber.
Merge pull request #23 from pmclanahan/validate-newsletters-subscribe

Check for valid newsletters before creating subscriber.
Remove non-existent newsletters from fixture

- Remove newsletters from fixture if they're not in ET
- Show the vendor_id in the admin so we notice next time if we have
  newsletters without vendor IDs.

Bug 841426
Fix deprecation error and add 500 template.
Merge pull request #24 from dpoirier/bad-newsletter-ids

Remove non-existent newsletters from fixture