[llvm-readobj] - Don't call `unwrapOrErr` in `findSectionByName`.

We have a `findSectionByName` helper that tries to find a section
by it name. It is used in a few places, but never tested.

I'd like to reuse this helper for a different place.
For this, I've changed it to return Expected<> and now it
doesn't use `unwrapOrErr` anymore. It also now a member of
Dumper class and might report warnings.

Differential revision: https://reviews.llvm.org/D84651