Improve error message for `augment.model_fit()` with survival models

[hfrick]

augment() should explain that it needs eval_time to be able to calculate predictions of survival probability.

library(censored)
#> Loading required package: parsnip
#> Loading required package: survival

proportional_hazards()%>%
  fit(Surv(time, status) ~ ., data = lung) %>% 
  augment(new_data = lung)
#> Error in `augment()`:
#> ! The `eval_time` argument is missing, with no default.

^{Created on 2024-01-16 with reprex v2.0.2}

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