feat(docx): differentiate no-file from not-ZIP (#3306)
**Summary**
The `python-docx` error `docx.opc.exceptions.PackageNotFoundError`
arises both when no file exists at the given path and when the file
exists but is not a ZIP archive (and so is not a DOCX file).
This ambiguity is unwelcome when diagnosing the error as the two
possible conditions generally indicate a different course of action to
resolve the error.
Add detailed validation to `DocxPartitionerOptions` to distinguish these
two and provide more precise exception messages.
**Additional Context**
- `python-pptx` shares the same OPC-Package (file) loading code used by
`python-docx`, so the same ambiguity will be present in `python-pptx`.
- It would be preferable for this distinguished exception behavior to be
upstream in `python-docx` and `python-pptx`. If we're willing to take
the version bump it might be worth considering doing that instead.
