Improve error message Union not allowed (#17769)
* Improve error message Union not allowed
* make style
* Update src/transformers/hf_argparser.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
