Added a Tk error dialog to run.py inform the user if the subprocess can't

connect to the user GUI process.  Added a timeout to the GUI's listening
socket.  Added Tk error dialogs to PyShell.py to announce a failure to bind
the port or connect to the subprocess.  Clean up error handling during
connection initiation phase.  This is an update of Python Patch 778323.

M NEWS.txt
M PyShell.py
M ScriptBinding.py
M run.py

Backport candidate.

Lib/idlelib/ScriptBinding.py

@@ -137,6 +137,8 @@ class ScriptBinding:
             return
         flist = self.editwin.flist
         shell = flist.open_shell()
+        if not shell:
+            return  # couldn't open the shell
         interp = shell.interp
         if PyShell.use_subprocess:
             shell.restart_shell()