Added check to get_pvalue()

[ddsjoberg]

What changes are proposed in this pull request?

• Improved error messaging in check_pvalue() when there is an error and the survfit object was not created with estimate_KM(). (Add check that 'survfit' object was created with estimate_KM() #373)

Did you include unit tests for the proposed change/bug fix (https://testthat.r-lib.org/)?
yes

If there is an GitHub issue associated with this pull request, please provide link.
closes #373

library(survival)
library(visR)

lung %>%
  survfit(Surv(time, status) ~ sex, data = .) %>%
  get_pvalue()
#> Error: There was an error calculating the p-values.
#> The 'survfit' object was not created with `visR::estimate_KM()`.
#> The the error will likely be resolved by re-estimating the 'survfit' object with visR.
#> Error in terms.formula(formula, "strata", data = data): object '.' not found

^{Created on 2022-05-28 by the reprex package (v2.0.1)}

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