No longer raise errors for missing message IDs.

This was causing failures due to an exception being raised
attempting to send a welcome message before adding the record to
master subscribers. Also we were trying to send a message to a user
that may well not have been in MS which would be an error.
Merge pull request #108 from pmclanahan/fix-send-message-no-exist-error

No longer raise errors for missing message IDs.
Count number of times the Firefox OS workaround in subscribe is used.
Merge pull request #109 from Osmose/count-fxos-workaround

Count number of times the Firefox OS workaround in subscribe is used.
Fix bug 1073657: Retry tasks with increasing delays 8 times.