[llvm-readelf/obj] - Improve error reporting when dumping group sections.

Our code that dumps groups has 3 noticeable issues:
1) It uses `unwrapOrError` in many places.
2) It doesn't allow reporting unique warnings, because the `getGroups` helper is not
   a member of `DumpStyle<ELFT>`.
3) It might just crash. See the comment for `StrTableOrErr->data() + Sym.st_name` line.

In this patch I am starting addressing these points.
For start I've converted one of `unwrapOrError` calls to a unique warning.

Differential revision: https://reviews.llvm.org/D91798