llvm-dwarfdump: Print warnings on invalid DWARF

llvm-dwarfdump was silent even when the format of DWARF was invalid
and/or llvm-dwarfdump did not understand/support some of the constructs.
This can be pretty confusing as llvm-dwarfdump is a tool for DWARF
producers+consumers development.

Review comments also by @dblaikie.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D104271