[BUG] symlink dead file under sync path breaks syncing

[michaelbjames]

Describe the bug
If a symlink to a dead file location is put under sync, maestral will give a FileNotFoundError, not sync the file (expected), but will no longer update changes to valid files (unexpected).
This is a problem because emacs uses lock file that likely link nowhere, thus using emacs on a file under sync halts syncing of that file util maestral is restarted.

To Reproduce
0. [inside sync path]

1. ln -s foo bar (where neither file already exists)
2. Observe the error:

Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 926, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/home/michael/.local/lib/python3.7/site-packages/maestral/monitor.py", line 1442, in upload_worker
    sync.apply_local_changes(events, local_cursor)
  File "/home/michael/.local/lib/python3.7/site-packages/maestral/monitor.py", line 659, in apply_local_changes
    success += [f.result()]
  File "/usr/lib/python3.7/concurrent/futures/_base.py", line 428, in result
    return self.__get_result()
  File "/usr/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/michael/.local/lib/python3.7/site-packages/maestral/monitor.py", line 189, in wrapper
    res = func(self, *args, **kwargs)
  File "/home/michael/.local/lib/python3.7/site-packages/maestral/monitor.py", line 738, in _apply_event
    self._on_created(event)
  File "/home/michael/.local/lib/python3.7/site-packages/maestral/monitor.py", line 816, in _on_created
    size1 = osp.getsize(path)
  File "/usr/lib/python3.7/genericpath.py", line 50, in getsize
    return os.stat(filename).st_size

Expected behaviour
While bar should not be synced, I would expect that file to be ignored and other changes to continue to propagate.

System (please complete the following information):

• Maestral version: 0.3.2
• OS: Debian bullseye
• Desktop environment: awesomewm
• Qt version: pyqt5: 5.13
• Python version: 3.7.4

[samschott]

Fixed and released in v0.4.0.