Add extension methods for IReturns to allow specifying return values exceptions on async methods in a synchronous way

I could use them so I wrote them. I hope you find them useful too, I can write those for more overloads if needed.

The changed formatting in the .csproj-file did not show up in my local diff, there's really just one line added (&lt;Compile Include="Language\IReturnsExtensions.cs" />). Sorry for any inconveniences.

Daniel