[llvm-readobj/elf] - Don't fail when dumping an archive with a member that can't be recognized.

Imagine we have an archive that has 3 objects in the following order:
<valid known object>,<unknown object> and <valid known object>.

Currently llvm-readelf/obj report an error and stops dumping in the middle.
This patch changes the error reported to warning.

Differential revision: https://reviews.llvm.org/D86771