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.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
BibEdit: only notifications on error

* Uses the new parameter available in BibTask to *only* send e-mail
  alerts if the upload task fails.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
BibEditMulti: only notifications on error

* Uses the new parameter available in BibTask to *only* send e-mail
  alerts if the upload task fails.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Merge pull request #78 from jalavik/bibtask-better-emails

Better e-mail notifications on BibEdit, BibEditMulti