Display the (more commonly accurate) message that the mgm server closed our connection
and that we'll retry connecting to it.


ndb/src/common/transporter/TransporterRegistry.cpp:
  Fix error message in case of mgm disconnecting before we finished asking it for dynamic ports
ndb/src/mgmapi/mgmapi.cpp:
  Only display a mgm protocol error in the case of the mgm server having disconnected.
  
  Change ndb_mgm_is_connected to actually poll the socket to see if it's still open.