Fail tests if the test data is not found

[oyilmaz-nvidia]

Describe the bug

A suggestion regarding tests: I am seeing some of the video.filtering tests such as tests/stages/video/filtering/test_clip_aesthetic_filter.py are printing test_video.mp4 not found but the tests pass even though they were not run. I think we should fail them.

[sarahyurick]

We are also skipping an image test because the data is not there yet: https://github.com/NVIDIA-NeMo/Curator/blob/main/tests/stages/image/io/test_image_reader.py#L203

[weijiac0619]

The "error message" seen in the test output (est_video.mp4 not found) was coming from VideoTask.validate(), which called os.path.exists and printed when the file wasn't found. However, Task.post_init calls validate() but ignores the return value, so construction succeeded and all test logic ran normally. changing print into log.warning here in this PR.#1614