Support for exception handling from unmanaged caller

[hez2010]

Closes #1571

Codegen example:

[UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvStdcall) })]
private static unsafe int Do_Abi_GetPoint_2(IntPtr thisPtr, global::Windows.Foundation.Point* __return_value__)
{
    global::AuthoringTest.BasicClass __this__ = default;
    global::Windows.Foundation.Point ____return_value__ = default;
    *__return_value__ = default;
    try
    {
        ____return_value__ = (__this__ = global::WinRT.ComWrappersSupport.FindObject<global::AuthoringTest.BasicClass>(thisPtr)).GetPoint();
        *__return_value__ = ____return_value__;

    }
    catch (Exception __exception__)
    {
        if (global::WinRT.ExceptionHelpers.TryHandleWinRTServerException(__this__, __exception__))
        {
            return 0;
        }
        global::WinRT.ExceptionHelpers.SetErrorInfo(__exception__);
        return global::WinRT.ExceptionHelpers.GetHRForException(__exception__);
    }
    return 0;
}

[jkotas]

        if (global::WinRT.ExceptionHelpers.TryHandleWinRTServerException(__this__, __exception__))
        {
            return 0;
        }
        global::WinRT.ExceptionHelpers.SetErrorInfo(__exception__);
        return global::WinRT.ExceptionHelpers.GetHRForException(__exception__);

I assume that this block is repeated a lot. You may want to turn into an API or a per-assembly helper to make the code smaller.