core: Elaborate on maintenance err msg (#862440)

VDS_CANNOT_MAINTENANCE_VDS_IS_NOT_RESPONDING_AND_IS_SPM raised some
questions with users and is now more elaborate, specifically, how to
overcome the error and switch to maintenance.

Change-Id: Ic5e0e3f85d2f1ff1a94b152d6d739308fb970113
Bug-Url: https://bugzilla.redhat.com/862440
Signed-off-by: Vered Volansky <vvolansk@redhat.com>