Check extended attributes support when adding account

Situation

Sentry Issue: NUXEO-DRIVE-4M8

FsMarkerException: 
  File "nxdrive\manager.py", line 424, in start_engines
  File "nxdrive\engine\engine.py", line 1068, in start

Could not start <Engine name='HOSTNAME', server_url='SERVER_URL', has_token=True, is_offline=False, uid='df3bc5ddb7f911eba5f548a4724eec29', type='NXDRIVE'>

Analysis

The exception was raised because the FS check was not done at the account addition. Then, when starting the app, the check is done and it fails. There is no warning shown to the user, only a Sentry event, when enabled.

Solution

Do the FS check when adding an account.