[Depr] raise_on_error kwarg with errors kwarg in astype#14878
 - [x] closes #14878 
 - [x] tests passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Please check that the entry in `whatsnew/v0.20.0.txt`. Unsure that the update was in the 
`_whatsnew_0200.deprecations` or `_whatsnew_0200.prior_deprecations` so put it in the
former.