http: Raise FrontendError if socket creation fails

This removes the stacktraces when two Mopidy instances are started with the
same hostname/port configuration.
http: Format "server running" log msg like MPD

By removing the http://... URL in the log output, we hopefully remove the
dubious connection between hostname/port configuration and the URL you'll
actually use to browse the Mopidy web server.

Example log output with this change:

    ...
    INFO     Starting Mopidy frontends: HttpFrontend, MpdFrontend
    INFO     HTTP server running at [::ffff:127.0.0.1]:6680
    INFO     MPD server running at [::ffff:127.0.0.1]:6600
Merge branch 'develop' into feature/http-startup

Conflicts:
	mopidy/http/actor.py
http: Move Mopidy request handlers to helper method
http: Split socket and server creation
http: Make event emitting testable
Merge pull request #784 from jodal/feature/http-startup

http: Improve error handling at HTTP server startup