plugins: Use g_win32_error_message for HRESULT to string conversion

Merged Seungha Yang requested to merge seungha.yang/gst-plugins-bad:win-error-unicode into master
We don't need to duplicate a method for HRESULT error code to string
conversion. This patch is intended to
* Remove duplicated code
* Ensure FormatMessageW (Unicode version) and avoid FormatMessageA
  (ANSI version), as the ANSI format is not portable at all.
  Note that if "UNICODE" is not defined, FormatMessageA will be aliased
  as FormatMessage by default.
Edited by GStreamer Release Monkey

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