Raise better error for missing subdatasets

[garlic-os]

Description

Changes the WeatherModel bbox accessor to raise a specific error when the weather model given does not contain any subdatasets.

Motivation and Context

The code has been crashing here sometimes with a list index out of bounds error while trying to open the first subdataset in the given file. For better debugging, this change makes this case raise a different error that better explains what has happened.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• X ] I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.