Begin paths with / in problem report.

Otherwise you get reports like:

(archive)
: directory shadows redirect "/ /foo 301"; remove the directory or use a 301! forced redirect instead

src/manifest.go

···
 	var report []string
 	for _, problem := range manifest.Problems {
 		report = append(report,
-			fmt.Sprintf("%s: %s", problem.GetPath(), problem.GetCause()))
+			fmt.Sprintf("/%s: %s", problem.GetPath(), problem.GetCause()))
 	}
 	return report
 }