Assertion instead of Error message while updating gardenlinux submodule

[toanju]

What happened:

Updated gardenlinux submodul.

What you expected to happen:

Nicer error messages instead of an assertion:

$ ./.build.sh
Traceback (most recent call last):
  File "/builder/./parse_features", line 223, in <module>
    main()
  File "/builder/./parse_features", line 75, in main
    feature_graph = read_feature_files(args.feature_dir)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builder/./parse_features", line 187, in read_feature_files
    assert os.path.isfile(f"{feature_dir}/{ref}/info.yaml"), f"feature {node} references feature {ref}, but {feature_dir}/{ref}/info.yaml does not exist"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: feature cloud references feature _legacy, but features/_legacy/info.yaml does not exist
Traceback (most recent call last):
  File "/builder/./parse_features", line 223, in <module>
    main()
  File "/builder/./parse_features", line 75, in main
    feature_graph = read_feature_files(args.feature_dir)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builder/./parse_features", line 187, in read_feature_files
    assert os.path.isfile(f"{feature_dir}/{ref}/info.yaml"), f"feature {node} references feature {ref}, but {feature_dir}/{ref}/info.yaml does not exist"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: feature cloud references feature _legacy, but features/_legacy/info.yaml does not exist
make: *** No rule to make target 'openstackbaremetal-metal3-orabos-amd64'.  Stop.

Would be great if this directly points out to feature _legacy is missing