fix #4351: vzdump: send mail if job-end hook fails with mailnotification=failure

fix #4351: vzdump: send mail if job-end hook fails with mailnotification=failure

Since all tasks succeeded, previously no mail was sent in that case.

Note that the error passed to $self->sendmail() is added to the
subject of the mail if it is a single line or the beginning of the
mail otherwise. Thus changing the mail slightly compared to previously
for the case where the job-start hook fails and the case where the
job-end hook fails with mailnotification=always. But can be considered
an improvement, because the user sees the error right away.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>