Merge: Opportunity: log errors to stderr and update message on failed event creation

Log all errors to stderr and remove prints on normal behavior.

Also update the error message, as the "meetup already exists" is now impossible because we dropped the uniqueness of name/date/location a while back.

This is related to #2186, where the file descriptor leak lead to bloated log files, a disk full and failed event creation by lack of space.

Pull-Request: #2187
Reviewed-by: Jean Privat <jean@pryen.org>