make attempts to stratify on a `Surv` object error better

[hfrick]

it'd be good to have a simple error message explaining that you can't stratify on a Surv object rather than have the error from survival bubble up.

library(rsample)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(survival)
data("lung")

lung %>% 
  mutate(surv = Surv(time, status)) %>% 
  initial_split(strata = "surv")
#> Error in Ops.Surv(x[hi], qs): Invalid operation on a survival time

^{Created on 2021-03-16 by the reprex package (v1.0.0)}

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