Update warnings to be more helpful to users after 2026 Naomi estimates

[rtesra]

• Improve error message when population data area level does not match the
  model area_level option. Previously this caused a non-informative
  stopifnot(!is.na(mf_model[["population_t1"]])) failure; now the error
  names both the model level and the population data level. (ETH, Debug 43)
• Improve ART duplicate-record error in artnum_mf(). Previously the error
  named model-level aggregated areas (which could mislead users to check the
  wrong rows); now it reports the exact input area_id, area name, age group,
  sex, quarter, and duplicate count for each offending row. (ETH, Debug 44)
• Add validation in select_naomi_data() to detect when ANC already-on-ART
  counts exceed total ANC positives. Previously this produced implausible ART
  coverage values or a non-informative downstream error; now it stops early
  with the affected area, area name, and year. (RWA, Debug 28)

[r-ash]

This looks good, I'm happy to merge this after we fix the build errors. I'll look into those errors, something failing when building the qs package which is a bit frustrating.

We should definitely check these messages display ok in the app once it is deployed. I think it should be fine, but we should check as they use lots of special characters for formatting the display.

[r-ash]

Nice, I like the |, I think they are a bit clearer than what we've tried before when raising errors about rows of data.