[llvm-readobj] - Refine the error reporting in LLVMStyle<ELFT>::printELFLinkerOptions.

[llvm-readobj] - Refine the error reporting in LLVMStyle<ELFT>::printELFLinkerOptions.

It is possible to:
1) Avoid using the `unwrapOrError` calls and hence allow to continue dumping even when
   something is not OK with one of SHT_LLVM_LINKER_OPTIONS sections.
2) replace `reportWarning` with `reportUniqueWarning` calls. In this method it is no-op,
   because it is not possible to have a duplicated warnings anyways, but since we probably
   want to switch to `reportUniqueWarning` globally, this is a good thing to do.

This patch addresses both these points.

Differential revision: https://reviews.llvm.org/D83131