sources.smscalls: propogate errors if there are breaking schema changes

[seanbreckenridge]

related to: karlicoss/HPI#289

Am unsure if CI here will fail till the return type from HPI is updated, but I think the change is small enough should be able to merge after publishing a release there...

Just to show works locally with new types:

$ mypy -p promnesia.sources.smscalls
Success: no issues found in 1 source file

$ hpi doctor -S promnesia.sources.smscalls
✅ OK  : promnesia.sources.smscalls                        
✅     - stats: {'index': {'index': {'count': 371, 'last': datetime.datetime(2023, 4, 14, 20, 6, 18, 459000, tzinfo=datetime.timezone.utc)}}}

[karlicoss]

I think mypy can complain about unnecessary None checks, but it's a custom flag which we don't have :)