py/vm: Use lowercase letter at start of exception message.

For consistency with all the other exception messages.

py/vm.c

@@ -1124,7 +1124,7 @@ unwind_return:
                             }
                         }
                         if (obj == MP_OBJ_NULL) {
-                            obj = mp_obj_new_exception_msg(&mp_type_RuntimeError, "No active exception to reraise");
+                            obj = mp_obj_new_exception_msg(&mp_type_RuntimeError, "no active exception to reraise");
                             RAISE(obj);
                         }
                     } else {