Ignore the unfixable grub error message when /boot is on zfs

Closes #292

Author: agross

handlers/main.yml

@@ -39,5 +39,10 @@
 - name: update-grub
   command: update-grub
   register: _pve_grub_update
-  failed_when: ('error' in _pve_grub_update.stderr)
+  failed_when: >-
+    'error' in (
+      _pve_grub_update.stderr_lines |
+      reject('equalto', '/usr/sbin/grub-probe: error: unknown filesystem.') |
+      join('\n')
+    )
   tags: skiponlxc