revise errors for workflow sets

[simonpcouch]

Fixes #160. Also splits the check into two cases, raising two different conditions.

If there are no fitted workflows in the workflowset, an error:

The supplied workflow set must be fitted to resamples with workflows::workflow_map() before being added to a data stack.

If only some of the workflows aren't fitted, a warning:

! Some elements of the supplied workflow set failed to evaluate with resamples.
i The workflows with ID `reg_lr` and `reg2_svm` will be excluded from the data stack.

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.