Cancer incidence: improve error for 'cervix' cancer type

[vsriram24]

Example code:

allStates = cancerprof::incidence_cancer(
  area = "WA",
  areatype = "hsa",
  cancer = "cervix",
  race = "All Races (includes Hispanic)",
  sex = "both sexes",
  age = "all ages",
  stage = "all stages",
  year = "latest 5 year average"
)

Error should be:

Error in `cancerprof::incidence_cancer()`:
! For this cancer type, sex must be females

However, error currently is:

Error in 'process_resp()':
! Invalid input, please check documentation for valid arguments.
Backtrace:
 1. cancerprof::incidence_cancer(...)
 2. cancerprof:::process_resp(resp, "incidence")