cppwinrt.h: Improve conversion of error information

[arthuraraujo-msft]

When using WIL with C++/WinRT with the cppwinrt.h header, the winrt_to_hresult_handler global handler is hooked from WIL directly into the C++/WinRT codebase.

The handler allows WIL exceptions to be understood by C++/WinRT and their HRESULTs extracted, but messages are lost in the way when going over the API boundaries because only the HRESULT is converted in winrt::to_hresult().

C++/WinRT makes use of Get and SetErrorInfo() to recover the messages when re-creating the exceptions on the caller side. The set is done inside winrt::hresult_error::to_abi() in winrt::to_hresult() on the server side, but only when winrt_to_hresult_handler is not hooked. The retrieval happens when re-creating the winrt::hresult_error on the caller.

If WIL were to call the to_abi() method in the WIL hook for winrt_to_hresult_handler, it would properly trigger the storage of the message and later on it could be retrieved when C++/WinRT creates a new winrt::hresult_error on the caller side.

[arthuraraujo-msft]

          Correct me if I'm wrong, but you seem to effectively be using C++/WinRT as a way to call `RoOriginateError`? There's already a header for that: [result_originate.h](https://github.com/microsoft/wil/blob/master/include/wil/result_originate.h).

Originally posted by @dunhor in #498 (comment)

Duncan's suggestion resolves this issue. I hadn't seen this before anywhere. Perhaps this could be added to the docs? Especially the C++/WinRT integration docs on the WIL wiki.