Don't claim error_report_err() always reports to stderr.  It actually
reports to the current monitor when we have one.

Clarify intended use of error_abort and error_fatal.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1454522628-28294-2-git-send-email-armbru@redhat.com>
Reviewed-by: NLluís Vilanova <vilanova@ac.upc.edu>