257249 missing trt start date

[gm-ebs-ext]

No description provided.

[ml-ebs-ext]

For anyone else lookint at this PR, here's how the error message looks like when a subject is missing the trt_start_date value:

I'm OK with the changes. Maybe the feedback would be more actionable if the messages mentioned also the name of the affected dataset (available in the subject_level_dataset_name argument to check_papo_call). Could you add that, @gm-ebs-ext?

I think I may be holding these new error messages to a higher standard than the pre-existing dv.papo messages because we've iterated quite a bit on the early error feedback user-friendliness in our other packages. Sorry for nitpicking!