main: Split main() function in two

By separating the teardown procedures for errors that happens before and after
actors are started, we get a lot less output when failing on config errors.
This makes the config errors the last lines printed by the `mopidy` command,
making them easier to spot.

Related to #467
Merge pull request #511 from jodal/feature/split-main-func

main: Split main() function in two