errors when collecting results.

One resampling and/or grid tuning is complete, and the results are collected using [`bind_rows()`](https://github.com/tidymodels/tune/blob/0f005416c778b94dd47bccf3df8b3b2b7d69a8f1/R/tune_grid_loop.R#L125). If there is a problem in doing this, the error message is very vague. 

We should do a better job handling this. 