YAML loading either needs simplification or better testing

See [this comment thread](https://github.com/canonical/craft-application/pull/20/files/88c186c8a772c0ad8f5a8cfeb8fe84d352729111#diff-e1faa3a7dda894734ce22e2d235c8e3a1fd7e0ad2ae234fff07ed0fe131af6d3).

The modifications made to the YAML loader may be more than is needed with the latest pyyaml, but I'm not confident enough to say so. If the questioned `except` clause is needed, we should have a test for it that has it raise that error, as right now the loader fails with the same issue before it gets to the dict constructor.

Start by reading the YAML loader changes in the linked pull-request diff and trace the failing path described in the issue. Determine whether the questioned exception handling is still needed with the current PyYAML behavior, then add a test that exercises the relevant error before the dictionary construction. Done means the loader is simplified or the behavior is covered by a focused test.

Written by the indexing model from the issue text.