improve axis error message

dune/microstructure/matrix_operations.hh

@@ -116,7 +116,7 @@ namespace MatrixOperations {
 						{         0,               0,  1.0}};
 			}
 			default:
-				DUNE_THROW(Dune::Exception, " axis not feasible. rotationMatrix is only implemented for 3x3-matrices.");
+				DUNE_THROW(Dune::Exception, " axis not feasible. rotationMatrix is only implemented for 3x3-matrices. Choose between 0: x-axis, 1: y-axis, 2: z-axis");
 		}
 	}