Crystal Report using multiple SQL Servers

Question

I have a report which get data from 4 different SQL Servers using database command getting data through SQL open query.
I had added a database command and getting data from 4 different sql servers and union it to display the reports. It is working fine in normal case.
The report is uploaded in Crystal Report server and scheduled to run automatically through CMC and the output is send through email to required person.
But if any of the SQL server is not available or connected, the report fails.
I want something through which I can check the status of Server/Database. If any of the server is fails I want to ignore that server and display the report using available servers.
As report is scheduled and run through report server, I have limited option to perform the server connection status through SQL.
I don't know how to do this, may be through some variables or parameters, or through some CMC option, any one having the solution please help me.