Log when starting letsencrypt endpoint + clearer errors

Running `gotosocial server` with the default configuration will try to
bind to :80 and listen for letsencrypt challenges, which will fail if
running as non-root (w/o capabilities), or if eg. nginx hogs the port.

When that happens, this should make it more obvious what's wrong.

internal/router/router.go

@@ -79,8 +79,9 @@ func (r *router) Start() {
 		// serve the http handler on the selected letsencrypt port, for receiving letsencrypt requests and solving their devious riddles
 		go func() {
 			listen := fmt.Sprintf("%s:%d", bindAddress, lePort)
+			logrus.Infof("letsencrypt listening on %s", listen)
 			if err := http.ListenAndServe(listen, r.certManager.HTTPHandler(http.HandlerFunc(httpsRedirect))); err != nil && err != http.ErrServerClosed {
-				logrus.Fatalf("listen: %s", err)
+				logrus.Fatalf("letsencrypt: listen: %s", err)
 			}
 		}()