[llvm/Object] - Make ELFObjectFile::getRelocatedSection return Expected<section_iterator>

It returns just a section_iterator currently and have a report_fatal_error call inside.
This change adds a way to return errors and handle them on caller sides.

The patch also changes/improves current users and adds test cases.

Differential revision: https://reviews.llvm.org/D69167

llvm-svn: 375408