output some error info if system fails

t/cp/gh-115-copy-into-dir.t

@@ -47,7 +47,9 @@ On Windows, cp a.txt b.txt fails at line 252 with message "cp: can not access B.
 
 subtest 'same directory' => sub {
 	my $rc = system $^X, $program_path, $filename, $second_filename;
-	is $rc, 0, 'system exited with 0';
+	is $rc, 0, 'system exited with 0' or diag(
+		"system failed:\n\t$!\n\t$^E"
+		);
 	ok -e $second_filename, "$second_filename exists";
 	};
 
@@ -59,7 +61,9 @@ Overall, this script has problems with case sensitivity, for example cp a.txt di
 
 subtest 'into directory' => sub {
 	my $rc = system $^X, $program_path, $filename, $subdir;
-	is $rc, 0, 'system exited with 0';
+	is $rc, 0, 'system exited with 0' or diag(
+		"system failed:\n\t$!\n\t$^E"
+		);
 	ok -e catfile( $subdir, $filename ), "$subdir/$second_filename exists";
 	};