iImplement the following return status from TGraph::Fit:

   The function return the status of the fit in the following form
     fitResult = migradResult + 10*minosResult + 100*hesseResult
   The fitResult is 0 is the fit is OK.
   The fitResult is negative in case of an error not connected with the fit.


git-svn-id: http://root.cern.ch/svn/root/trunk@18608 27541ba8-7e3a-0410-8455-c3a389f83636