Clean up error message for unmounting volume that is not actually mou…

…nted

valdi/cli/volume_manager.py

@@ -78,5 +78,5 @@ def unmount_volume(mountpoint):
                 check=True
             )
         except subprocess.CalledProcessError as e:
-            print(f'Error unmounting volume: {e.stderr}')
+            print(f'Error unmounting volume{f": {e.stderr}" if e.stderr is not None else ""}')
             sys.exit(1)