Do not apply subsequent updates if one failed.

Sequel::Rollback gets caught by Sequel.transaction block so all
subsequen updates were applied. Now we just mark update as failed and re-raise
the exception to end the process.

Apply task won't run and will send an error message if there are any
failed updates.
Merge pull request #68 from alphagov/stop_synchronizer_on_failure

Do not apply subsequent updates if one failed.