Change some os.IsNotExist to errors.Is(err, os.ErrNotExist) for non-os errors.

os.IsNotExist doesn't unwrap errors. errors.Is does.

The ioutil.ReadFile ones happened to be fine but I changed them so
we're consistent with the rule: if the error comes from os, you can
use os.IsNotExist, but from any other package, use errors.Is.
(errors.Is always would also work, but not worth updating all the code)

The motivation here was that we were logging about failure to migrate
legacy relay node prefs file on startup, even though the code tried
to avoid that.

See golang/go#41122

cmd/derper/derper.go

@@ -63,7 +63,7 @@ func loadConfig() config {
 	}
 	b, err := ioutil.ReadFile(*configPath)
 	switch {
-	case os.IsNotExist(err):
+	case errors.Is(err, os.ErrNotExist):
 		return writeNewConfig()
 	case err != nil:
 		log.Fatal(err)