spec-parser run reports errors but exits with `0`

The current spec parser reports an error but does still exit with EXIT_SUCCESS. See for example https://github.com/spdx/spdx-3-model/actions/runs/7118823989/job/19382571564#step:5:25 with the log:

 > Run cd spec-parser
WARNING : External property restrictions aren't yet handled properly, they are added to the description of the class.
WARNING : skipping ../model/Core/Properties/_property.md
WARNING : skipping ../model/Core/Vocabularies/_vocab.md
WARNING : External property restrictions aren't yet handled properly, they are added to the description of the class.
WARNING : External property restrictions aren't yet handled properly, they are added to the description of the class.
WARNING : External property restrictions aren't yet handled properly, they are added to the description of the class.
WARNING : External property restrictions aren't yet handled properly, they are added to the description of the class.
WARNING : Overwriting out_dir `/tmp/spec-parser.out`
ERROR   : Invalid Nature attribute in metadata `None`

Start by reproducing the spec-parser command using the linked GitHub Actions log, then trace how the Invalid Nature attribute in metadata None error is handled. The fix is done when this error causes the process to exit with a nonzero status while successful parsing still exits with 0; add or update a test if the repository provides one.