Encapsulation of eaddrinuse error when starting cowboy

Instead of the deeply nested error structure returned by cowboy, just return `{:error,:eaddrinuse}`.

This is to facilitate resolving https://github.com/phoenixframework/phoenix/issues/180, and was suggested there.
Let's move this test to `test/plug/adapters/cowboy/conn_test.exs`. It is not the ideal one but there we at least start the server already on setup.