[TC] Problem: Bad Variant Access errors are not explicit

[nico34638]

Hello,

I am a Tiger Maintainer 2024 at Epita,

This year we step into a recurrent issue with bison. Students
almost always faced a bad variant acces error while mismatching
types. The problem with this is that they have no way to discriminate
the source of the error.

It could be a good idea to have some security and some checkings
around variant access.