Add error message for invalid label schema

[Oufattole]

When labels don't follow the meds label-schema, users currently receive an unhelpful error message:

Clearing lock due to Exception '<=' not supported between instances of 'int' and 'NoneType' at ...

This occurs in the task caching stage when prediction_time in a task is null.

Proposed solution:

1. Add a check to validate that labels conform to the meds-schema
2. Implement a clear error message when invalid labels are detected

Specifically, the check and error should be right after this line where labels are read.