Improve nuScenes dataset verification messaging

[AkilanManivannanak]

Summary

• add clearer file-missing error handling to assert_download.py
• print dataset root and version before verification
• report successful verification with checked file counts
• add unit tests for the new file existence helper

Motivation

The current verification script works, but when setup issues occur it is hard to see what the script is checking and why it failed. This change keeps the existing workflow intact while making dataset validation output more informative.

Testing

• python -m py_compile python-sdk/nuscenes/tests/assert_download.py
• cd python-sdk
• python -m unittest nuscenes.tests.test_assert_download