grid,manager: close xmlrpc.ServerProxy sockets
by using it getSupervisorRPC in a context manager which on python 3 automatically closes. This needs fix from https://github.com/Supervisor/supervisor/issues/1184 we assume that this fix is only on python 3. On python2, we keep this same behavior of not closing socket explicitly and leaving it to destructors. Also re-raise a few error cases that were ignored.
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