keep_going = TRUE fails for formatted targets

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bug still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Reproducible example

library(drake)
plan <- drake_plan(
  x = target(stop(123), format = "fst_tbl"),
  y = target(stop(123), format = "fst_tbl")
)
make(plan, keep_going = TRUE)
#> ▶ target x
#> x fail x
#> Warning: You selected fst_tbl format for target x, so drake will convert it from
#> class c("simpleError", "error", "condition") to a tibble.
#> Error in as.data.frame.default(value, stringsAsFactors = FALSE): cannot coerce class 'c("simpleError", "error", "condition")' to a data.frame

^{Created on 2020-03-06 by the reprex package (v0.3.0)}