User-Reported Defect: Improve error handling in SaveBaseSettings

[MeWu-IDM]

We received support request from @Nyerere
Describe the defect
When Task target populations does not exist in Lookup, user should get a very clear error message about how to fix it

To reproduce
Steps to reproduce the behavior:

1. Create a tasks sheet with label "xxx" that is not defined in lookup
2. Run the scenario that use this task sheet

Expected behavior
A clear error message to let user know how to add population label properly

Actual behavior
https://github.com/InstituteforDiseaseModeling/PACE-HRH/blob/main/pacehrh/R/pace_exp_control.R#L95-L100
(1) it should not be warning but error
(2) message can be improved to show which scenario and some suggestions such as, "did you forget to add the label definition..."