Checks on availability of 'sphinx_exercise_registry' attribute

[AakashGfude]

Describe the problem

In some places in the code, it is assumed that sphinx_exercise_registry will always be present like in
https://github.com/executablebooks/sphinx-exercise/blob/master/sphinx_exercise/post_transforms.py#L213
, which leads to errors like

found in lecture-datascience.myst build: https://github.com/QuantEcon/lecture-datascience.myst/runs/4439388922?check_suite_focus=true

Link to your repository or website

No response

Steps to reproduce

1. Clone https://github.com/QuantEcon/lecture-datascience.myst locally
2. run 'jb build lectures --builder==pdflatex' with sphinx-exercise==0.3.0

The version of Python you're using

No response

Your operating system

No response

Versions of your packages

sphinx-exercise: 0.3.0

Additional context

No response

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