upkeep: improve output validation in `format_scenarios_for_p4b.R`

[jacobvjk]

as mentioned here #58
output validation could be improved.
open: does it make sense to add functions to pacta.data.validation package or should they go directly into the format_p4b and format_p4b_ei functions?
I would tend to add mostly type checks, as the levels are implicitly already checked in the scenario preparation step

[cjyetman]

I see two options:

1. do proper validation of whatever is necessary in the code that exports them
2. add a validate_p4b_scenario_data() type function to pact.data.validation

honestly, probably both are legitimate and worthwhile, though option 2 could possibly serve both purposes