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<apex:pageMessage summary="Your custom message what you want to display ." severity="info" strength="3" />
Hope the below links will help you.
http://salesforce.stackexchange.com/questions/11124/customize-standard-error-message-on-the-visualforce-page
http://www.sfdcpoint.com/salesforce/show-error-message-visualforce-page/
P.S - Please mark this as solution by selecting it as best answer if this solves your problem.
Thanks
If you want to add custom message in vf page directly then add below component and modify the message and you can render with your condition . you can chnage severity according to your need .
For more information you please check below link .
http://www.salesforce.com/us/developer/docs/pages/Content/pages_compref_pageMessage.htm
Thnaks
Manoj