tools: Make "No transport ..." error more understandable

Makes current error message "No transport is enabled, nothing to do"
more understandable to users:

  "No transport is enabled, please enable SMTP (using MAIL_SERVER option)
  or SNMP (using SNMP_MANAGERS option)."

Change-Id: I8aa720ff3d15ed45ec59035926bd2db8b7f2d020
Bug-Url: https://bugzilla.redhat.com/1136026
Signed-off-by: Martin Perina <mperina@redhat.com>