Re-add error handling

Author: joshmoore

zarr/n5.py

@@ -311,7 +311,11 @@ def group_metadata_to_n5(group_metadata):
 
 def group_metadata_to_zarr(group_metadata):
     '''Convert group metadata from N5 to zarr format.'''
-    group_metadata.pop('n5')
+    try:
+        group_metadata.pop('n5')
+    except KeyError:
+        # This only exists at the top level
+        pass
     group_metadata['zarr_format'] = ZARR_FORMAT
     return group_metadata