Check in DoFHandler::renumber_dofs() that DoFs are distributed

This adds several Asserts to check that DoFs are distributed
before calling renumber_dofs() for active or level DoFs. Note
that we now also require that you call distribute_dofs() before
distribute_mg_dofs().

Let's wait until we complete the discussion in https://groups.google.com/d/topic/dealii/G3Xp-Uv72_M/discussion
While we're here, can we capitalize the first word of the error message as we do everywhere else?