[obj2yaml] - Cleanup error reporting (remove Error.cpp/.h files)

This removes Error.cpp/.h files from obj2yaml.
These files are not needed because we are
using `Error`s instead of error codes widely and do
not need a logic related to obj2yaml specific
error codes anymore.

I had to adjust just a few lines of tool's code
to remove remaining dependencies.

Differential revision: https://reviews.llvm.org/D86536