Broken pipe error after interrupt

[aivarannamaa]

• Run script with import turtle; turtle.exitonclick().
• Try to interrupt it (needs waiting and serveral Ctrl+C-s)
• Interrupt idle prompt
• KeyboardInterrupt appears
• Press ENTER

Got

Traceback (most recent call last):
  File "/home/aaa/python_stuff/thonny/thonny/shell.py", line 1307, in _submit_input
    get_runner().send_command(
  File "/home/aaa/python_stuff/thonny/thonny/running.py", line 262, in send_command
    response = self._proxy.send_command(cmd)
  File "/home/aaa/python_stuff/thonny/thonny/running.py", line 1040, in send_command
    self._send_msg(cmd)
  File "/home/aaa/python_stuff/thonny/thonny/running.py", line 1047, in _send_msg
    self._proc.stdin.write(serialize_message(msg) + "\n")
BrokenPipeError: [Errno 32] Broken pipe

[aivarannamaa]

Each subsequent ENTER gives the same. The runner doesn't understand that backend has closed.

[aivarannamaa]

Fix this before #2422