Fixed lookup sample errors

[arnvptl]

This PR enhances the lookup_sample() method in malariagen_data/anoph/sample_metadata.py to provide more informative error messages when invalid sample IDs or indices are provided.

Changes:

• Catch KeyError and IndexError exceptions and raise ValueError with helpful messages instead.
• For non-existent sample IDs: "Sample 'ID' not found. Available samples can be found by calling sample_metadata()."
• For invalid indices: "Sample index X is out of bounds. Valid indices are 0 to Y."
• Added comprehensive tests in tests/anoph/test_sample_metadata.py to verify error handling.

Motivation:
The original raw KeyError was unhelpful for users. This improves the developer experience by providing actionable feedback.

Fixes #[1078].