Network: Print used port on startup and change default to 6742

Also print an error if server can't be started

1337 is already used by razer's rest server
Port idea by @bahorn (6742 = ORGB on a phone numpad)
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f