BibSched: email-logs-on-error parameter

• Adds a new parameter email-logs-on-error available to all BibTasks that allows you to activate an e-mail alert to task submitter if the task should fail (and only if it fails). E-mail alerts for successful tasks are still available through the email-logs-to option.

• Improves the readability of the standard e-mails sent with --email-logs-on-error or --email-logs-to options. (PR #2205)

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>