Ignore missing temporary directories when booting.

Otherwise, removing your `app/tmp` and `app/routes` folders prior
to booting (e.g., if you add them to your .gitignore and use Travis)
will cause unnecessary FATAL error messages. Addresses
revel/revel#908.

harness/build.go

@@ -173,7 +173,9 @@ func cleanDir(dir string) {
 	tmpPath := path.Join(revel.AppPath, dir)
 	f, err := os.Open(tmpPath)
 	if err != nil {
-		revel.ERROR.Println("Failed to clean dir:", err)
+		if !os.IsNotExist(err) {
+			revel.ERROR.Println("Failed to clean dir:", err)
+		}
 	} else {
 		defer f.Close()
 		infos, err := f.Readdir(0)