arb looses GUI when unable to acquire socket

[westram]

reproduce:

• arb_ntree demo.arb
• start another ntree in same context (e.g. interrupt and detach in gdb, then run again)
• message

  THIS PROGRAM HAS PROBLEMS TO OPEN INTERCLIENT COMMUNICATION:
  Reason: ARB_DB_SERVER_ERROR: Socket ':' already in use
  (maybe there is already another server running)
  You cannot use any EDITOR or other external SOFTWARE from here.
  Advice: Close ARB again, open a console, type 'arb_clean' and restart arb.
  Caution: Any unsaved data in an eventually running ARB will be lost.
  

  pops up.
• arb_ntree has no window, but waits for input

Notes:

• It's ok, that arb_ntree does not terminate instantly - otherwise the error message would not be readable.
• In the past the main window opened up normally in that case
• happens in gtk and motif

related: #554