Backport #5525 Fix the Let's Encrypt handler

[gregkare]

• 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