Opt-in inheritance from std::exception for winrt::hresult_error

[JaiganeshKumaran]

Related to #1336, #1129 and #207.

Currently winrt::hresult_error does not inherit from std::exception. When writing cross-platform applications that may use C++/WinRT internally on Windows, one would except that they could catch std::exception to catch all exceptions with error message. Today I need an extra layer to convert an HRESULT to the appropriate std::exception-derived type so the calling code, for example, can just catch std::invalid_argument instead of needing to know about winrt::hresult_invalid_argument.

For the error message, I would recommend lazily converting the hstring into a UTF-8 encoded std::string if winrt::hresult gets inherited from std::exception.

[kennykerr]

This was considered and rejected years ago. Here are some of the reasons:

https://github.com/microsoft/cppwinrt/issues/207#issuecomment-308500627

[JaiganeshKumaran]

It would also force calls into the CRT from both constructor and destructor - all to produce a message that lacks much of the richness of WinRT's error information.

You do not have to use the constructor and in fact, std::exception does not have a constructor taking error message (Microsoft's implementation does but that is non-standard) - only std::runtime_error and logic_error do. Instead, override the virtual what method and lazily convert the error message to UTF-8. This should not have much overhead.

Also the constructor of std::exception is just an inline function (at least in MSVC) so no calls into the CRT is required.

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