[notification] avoid leaking cnxsets (closes #3243810)
When sending notifications, we get each recipient as either an email
address or a CWUser. In the latter case, we create a temporary session
for that user and use it to send the mail. However, if we later decided
to not send the mail after all, we'd leak the session and its cnxset.
Add a try block inside the loop to make sure the temporary sessions are
closed properly.