Saved search email improvements [MJ-518]

Log exceptions in daily task
Don't send emails to people with no searches
Hide send_if_none for saved search digests [MJ-518]

Automatically detect if an email should be sent based on the presence of feed items
Merge pull request #318 from DirectEmployers/saved-search-reverse-error__MJ-518

Saved search email improvements [MJ-518]