Backported #5525 Fix the Let's Encrypt handler (#5527)

Backported #5525 Fix the Let's Encrypt handler (#5527)

* Fix the Let's Encrypt handler by listening on a valid address

Also handle errors in the HTTP server go routine, return a fatal error
when something goes wrong.

Thanks to @gbl08ma for finding the actual bug

Here is an example of the error handling:

    2018/12/11 14:23:07 [....io/gitea/cmd/web.go:87 func1()] [E] Failed to
    start the Let's Encrypt handler on port 30: listen tcp 0.0.0.0:30: bind:
    permission denied

Closes #5280

* Fix a typo