Fix TaxonomyReadingException message not displaying

[courtneypacheco]

Resolves #75

To ensure the exception message gets printed, we need to either define an __init__ function in the custom TaxonomyReadingException class OR add pass.

I opted for the latter approach and added unit tests to confirm the exception message gets printed/returned.

[nimbinatus]

LGTM; thank you!

[nathan-weinberg]

@instructlab/schema-maintainers once this is merged please cut a 0.4.2 so we can unblock the XL E2E CI job

[bjhargrave]

please cut a 0.4.2

Released: https://pypi.org/project/instructlab-schema/0.4.2/

[reidliu41]

hi @courtneypacheco seems still cannot fix the failed reading from text, e.g.
$ head /xx/instructlab/taxonomy/compositional_skills/engineering/qna.yaml

version: 2
task_description: 'Teach the model who is testlin.'
created_by: testliu
seed_examples:xx      # <<<<<<<<<<<<<<<
  - question: Who is Test Lin?
 
$ ilab taxonomy diff
compositional_skills/engineering/qna.yaml
Reading taxonomy failed with the following error:  # not display

it might still need to pass the message e.g.

   437	        except Exception as e:
   438	            raise TaxonomyReadingException(f"{e}") from e

$ ilab taxonomy diff
compositional_skills/engineering/qna.yaml
Reading taxonomy failed with the following error: while scanning a simple key
  in "<unicode string>", line 4, column 1:
    seed_examples:xx
    ^
could not find expected ':'
  in "<unicode string>", line 5, column 13:
      - question: Who is Test Lin?
                ^