Add additional information in TGClient::GetColorByName after printing the Warning;

"GetColorByName", "couldn't retrieve color xxxx"
 This problem typically appears when running ROOT from
 an X terminal with not enough memory and another application
 like Netscape is already running
 ====> Kill Netscape and start again ROOT

I am tired to always answer the same question.


git-svn-id: http://root.cern.ch/svn/root/trunk@4295 27541ba8-7e3a-0410-8455-c3a389f83636