Thrift TException does not properly define virtual what() method.

Summary:
Clients would elect to call the base class what() method instead of the derived version.  This results in
less than usable exceptions for debugging.

Reviewed By: Karl * mcslee

Test Plan:
Tested with local application in development.

Revert Plan:
revertible.


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@664887 13f79535-47bb-0310-9956-ffa450edef68

lib/cpp/src/Thrift.h

@@ -23,7 +23,7 @@ public:
 
   ~TException() throw() {}
 
-  const char* what() {
+  const char* what() const throw() {
     if (message_.empty()) {
       return "Default TException.";
     } else {