[yaml2obj/ObjectYAML] - Cleanup the error reporting API, add custom errors handlers.

[yaml2obj/ObjectYAML] - Cleanup the error reporting API, add custom errors handlers.

This is a continuation of the YAML library error reporting
refactoring/improvement and the idea by itself was mentioned
in the following thread:
https://reviews.llvm.org/D67182?id=218714#inline-603404

This performs a cleanup of all object emitters in the library.
It allows using the custom one provided by the caller.

One of the nice things is that each tool can now print its tool name,
e.g: "yaml2obj: error: <text>"

Also, the code became a bit simpler.

Differential revision: https://reviews.llvm.org/D67445

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371865 91177308-0d34-0410-b5e6-96231b3b80d8