Give a clue why the FLUENT dat file may not have opened.

The base file name for the cas and dat files must be the same, so say
so in the error message if the dat file can't be opened.

IO/Geometry/vtkFLUENTReader.cxx

@@ -541,6 +541,8 @@ bool vtkFLUENTReader::OpenDataFile(const char *filename)
 
   if (this->FluentDataFile->fail())
   {
+    vtkErrorMacro("Could not open data file " << dfilename << "associated with cas file "
+                  << filename << ". Please verify the cas and dat files have the same base name.");
     return false;
   }
   else