[llvm-readobj] - Introduce warnings for cases when unable to read strings from string tables.

Currently we have no dedicated warnings, but we return error message instead of a result.
It is generally not consistent with another warnings we have.

This change was suggested and discussed here:
https://reviews.llvm.org/D77216#1954873

This change refines error messages we report and also I had to update the API
to implement it.

Differential revision: https://reviews.llvm.org/D77399