Print error message in diff_files if any of the two files to diff does not exist

client/mysqltest.cc

@@ -3335,6 +3335,14 @@ void do_diff_files(struct st_command *command)
                      sizeof(diff_file_args)/sizeof(struct command_arg),
                      ' ');
 
+  if (access(ds_filename.str, F_OK) != 0)
+    die("command \"diff_files\" failed, file '%s' does not exist",
+        ds_filename.str);
+
+  if (access(ds_filename2.str, F_OK) != 0)
+    die("command \"diff_files\" failed, file '%s' does not exist",
+        ds_filename2.str);
+
   if ((error= compare_files(ds_filename.str, ds_filename2.str)))
   {
     /* Compare of the two files failed, append them to output

mysql-test/mysql-test-run.pl

@@ -3486,13 +3486,22 @@ sub report_failure_and_restart ($) {
   if ( !defined $tinfo->{logfile} )
   {
     my $logfile= $path_current_testlog;
-    if ( defined $logfile and -f $logfile )
+    if ( defined $logfile )
+    {
+      if ( -f $logfile )
       {
 	# Test failure was detected by test tool and its report
 	# about what failed has been saved to file. Save the report
 	# in tinfo
 	$tinfo->{logfile}= mtr_fromfile($logfile);
       }
+      else
+      {
+	# The test tool report didn't exist, display an
+	# error message
+	$tinfo->{logfile}= "Could not open test tool report '$logfile'";
+      }
+    }
   }
 
   after_failure($tinfo);