[Refactoring] Improve FileNotFoundError message in PointNavDatasetV1

[anlorla]

Motivation and Context

When either data_path or scenes_dir is missing, PointNavDatasetV1.get_scenes_to_load raises a FileNotFoundError that only prints the dataset directory.
This can mislead users into focusing on data_path while the real issue is often scenes_dir.

This PR refactors the error message to display both data_path (expanded with split) and scenes_dir to improve debuggability.

How Has This Been Tested

• Locally set a valid data_path and an invalid scenes_dir; verified the new message includes both paths and clearly indicates the missing one.
• No functional behavior change; only error text is affected.

Types of changes

• [Refactoring] Improve clarity of error messages without altering functionality.

Checklist

• My code follows the code style of this project.
• I have updated the documentation if required. (N/A)
• I have read the CONTRIBUTING document.
• I have completed my CLA (will sign if prompted).
• I have added tests to cover my changes if required. (N/A, message-only change)

Before / After

Before
FileNotFoundError: Could not find dataset file <dataset_dir>

After

FileNotFoundError: Dataset paths not found.
data_path: <expanded data_path>
scenes_dir: <scenes_dir>```