_dbus_server_new_for_tcp_socket: fix error handling

_dbus_server_new_for_tcp_socket: fix error handling

This is one of the patches from Maemo's dbus package. It seems to do all
of:

* fix some documentation
* remove unreached code to delete/free the nonce file from
  _dbus_server_new_for_socket - doing that on failure violates
  least-astonishment anyway
* in _dbus_server_new_for_tcp_socket, never fail without setting @error
* if we fail after creating the nonce file, delete it
* if we fail after allocating the nonce file struct, free it

Origin: vendor, Maemo
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33128
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>