We should support notification system or at least not fail when notification system is installed.

Currently we faul with:

Nov 27 11:20:28 work-mox updater-supervisor: Traceback (most recent call last):
   File "/usr/bin/updater-supervisor", line 11, in <module>
    load_entry_point('svupdater==1.2.1', 'console_scripts', 'updater-supervisor')()
   File "/__main__.py", line 103, in main
   File "/_supervisor.py", line 177, in run
   File "/notify.py", line 135, in notifier
   File "/subprocess.py", line 339, in call
   File "/subprocess.py", line 800, in __init__
   File "/subprocess.py", line 1551, in _execute_child
 FileNotFoundError: [Errno 2] No such file or directory: 'notifier': 'notifier'