Guard directory check call

[marySalvi]

When we check if files are directories (and possibly elsewhere), we should guard that call. I had a dead mount point that caused the program to crash. This would probably be change from

self.child_directories = [
            child for child in directory.iterdir() if child.is_dir() and os.access(child, os.R_OK)
        ]

to

self.child_directories = []
for child in directory.iterdir():
    try:
        if child.is_dir() and os.access(child, os.R_OK):
            self._child_directories.append(child)
    except Exception OSError:
        pass

more generally, we should guard exceptions so as to keep the gui running.