update error message when age tracking is required

[adamhb]

When age tracking is required (but not switched on) fates raises an error flag instructing the user to set "hlm_use_cohort_age_tracking" to "true". The new namelist option for this is "use_fates_cohort_age_tracking". The name in the error message just needs to be updated.

[JessicaNeedham]

Good find, thanks @adamhb. This will also need to be updated in the warning about using age tracking with inventory init.