Handle absence of LinuxFileChooser backend
Handle absence of LinuxFileChooser backend

Currently this crashes with:
```
   File "/home/cheaterman/.local/lib/python3.7/site-packages/plyer/platforms/linux/filechooser.py", line 251, in _file_selection_dialog
     desktop_override = desktop
 NameError: name 'desktop' is not defined
```
when it should actually crash with:
```
   File "/home/cheaterman/.local/lib/python3.7/site-packages/plyer/platforms/linux/filechooser.py", line 254, in _file_selection_dialog
     raise OSError("No back-end available. Please install one.")
 OSError: No back-end available. Please install one.
```

Thanks :-)
kivy/plyer,kivy/plyer,kivy/plyer