gluster-nagios-monitoring: Added date validation error message on trends tab

Displays a error popup when the user selects To date
greater than From Date to show trends.

Change-Id: Id84d02203cd94a780303c5e802dba82e3db500ce
Bug-Url: https://bugzilla.redhat.com/1112183
Signed-off-by: Karnan TC <kchidamb@redhat.com>