feat: Add warning for missing import

[arwedus]

This warning is shown by sphinx build now when the "fullname" in the autoapi directive cannot be found in the imports, e.g. .. autoapiclass:: autoapi.mappers.python.objects.PythonPythonMapper give:

reading sources... [100%] reference/templates
WARNING: Failed to import class 'autoapi.mappers.python.objects.PythonPythonMapper' [autoapi.import]

Closes #419

[AWhetter]

Apologies for taking so long to look at this.
In addition to my other comments, please could you add a test for this. You'll also need to use towncrier to add a changelog entry.

[AWhetter]

Closing due to lack of follow up. Please feel free to resubmit if you can fix the tests and address the comments.

[arwedus]

Hi @AWhetter , I'm picking this up again because we lost our "missing import" warnings again after an update to Autoapi 😄

You mentioned a missing towncrier entry, but https://sphinx-autoapi.readthedocs.io/en/latest/maintenance/release-process.html does not mention towncrier anywhere. Can you be more specific?

I'll also add a test.