Please Enable TestingHelpers to throw Exceptions

[Rainer1]

It quite some scenarios it is necessary to ensure by UnitTests, that the written code handles Exceptions correctly which are thrown by the file system.
For this reason it would be great to be able to define a Exception to be thrown, when a specific Method Is called.
Example Similar to many Mocking Frameworks:
MockFileSystem.File.Delete.WhenCalled( (file) => throw new IOException(message))

[fgreinacher]

Duplicate of #168 - consider leaving your thumbs up there to show support :)