Be more clear that check-incremental outputs are errors

Author: kubamracek

utils/check-incremental

@@ -92,14 +92,14 @@ def main():
         # This is the most important check: is the output file exactly the
         # same.
         if reference_md5 != second_md5:
-            sys.exit("non-determinism when generating: " + output_file +
+            sys.exit("ERROR: non-determinism when generating: " + output_file +
                      "\ncommand line:\n" + " ".join(new_args))
 
         # This is the bonus check: does the compiler not re-write the output
         # file. (For compilations < 1sec this check may succeed even if the
         # file was overwritten).
         if compare_time and reference_time != second_time:
-            sys.exit("file re-written: " + output_file)
+            sys.exit("ERROR: file timestamp was re-written: " + output_file)
 
 
 if __name__ == '__main__':