Assign custom classes to errors and warnings

[sebffischer]

With mlr-org/mlr3misc#133 we now get the ability to assign custom classes to our conditions.
We should do this for those conditions that users might want to disable.
For those, the roxygen section Errors and Warnings should list the error class and describe the condition, see e.g.,

mlr3/R/benchmark_grid.R

Lines 18 to 19 in fd6b3ea

	#' @section Errors and Warnings:
	#' * `varying_predict_types`: This warning will be thrown if the learners have different `predict_type`s.

.