Use alternative method to surface errors from callr::r_safe()

tests/testthat/helper-session.R

   dir.create(empty_wd)
   owd <- setwd(empty_wd)
   on.exit({setwd(owd); unlink(empty_wd, TRUE)})
-  callr::r_safe(.f, args)
+  args$.f <- .f
+  res <- callr::r_safe(function(.f, ...) {
+    tryCatch(
+      list(result = .f(...), error = NULL),
+      error = function(e) list(result = NULL, error = e$message)
+    )
+  }, args)
+  if (!is.null(res$error)) {
+    stop(res$error)
+  } else {
+    res$result
+  }
 }

tests/testthat/test-themes.R

       options(warn = 2)
       xaringanthemer::style_xaringan(text_color = "rgb(100, 100, 100)", background_color = "white", outfile = NULL)
     }),
-    regexp = "Colors.+used by.+theme_xaringan",
-    class = "callr_status_error"
+    regexp = "Colors.+used by.+theme_xaringan"
   )
 })