s:EndTermDebug should only be called when exiting if the debugger started
without error, otherwise the plugin breaks.
Vim handles this by using job_setoptions to set the on_exit callback to
s:EndTermDebug after startup succeeds. However, Nvim does not have such
functionality; instead; use s:starting to mimic this behaviour.
Also, introduce s:running to fix s:CheckGdbRunning; it did not work correctly
due to the "[Process exited X]" message keeping the job's channel alive (though
the stream is closed). This means nvim_get_chan_info cannot be used to check if
the debugger has exited, as it may still return a non-empty dict.
315858bf67
