Generalize "Cancel Compute" mechanism to be able to report

"Computation canceled" instead of "Algorithm failed" as failure reason

+      CASE2TEXT( COMPERR_CANCELED      );

src/SMESHGUI/SMESHGUI_ComputeDlg.cxx

@@ -345,6 +345,7 @@ namespace SMESH
       CASE2TEXT( COMPERR_EXCEPTION     );
       CASE2TEXT( COMPERR_MEMORY_PB     );
       CASE2TEXT( COMPERR_BAD_SHAPE     );
+      CASE2TEXT( COMPERR_CANCELED      );
     case SMESH::COMPERR_ALGO_FAILED:
       if ( strlen(comment) == 0 )
         text = QObject::tr("COMPERR_ALGO_FAILED");