[llvm-readobj] - Refine error reporting in MipsGOTParser<ELFT> helper.

This is a follow-up for D83225. This does the following:
1) Adds missing tests for existent errors.
2) Stops using `unwrapOrError` to propagate errors to caller.
   (I am trying to get rid of all `unwrapOrErr` calls in the llvm-readelf code).
3) Improves error messages reported slightly.

Differential revision: https://reviews.llvm.org/D83314