Make error messages during PPT processing clearer. (#6980)
### What problem does this PR solve?
Sometimes a slide may trigger a Proxy error (ArgumentException:
Parameter is not valid) due to issues in the original file, and this
error message can be confusing for users.
### Type of change
- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [x] Other (please describe):