Improved exception for GuardClauseAssertion

After this PR, when `GuardClauseAssertion` verification fails for a method that returns `Task` or `Task<T>` including those marked `async`, the following exception message is returned:

> A Guard Clause test was performed on a method that returns a Task, Task\<T\> (possibly in an 'async' method), but the test failed. See the inner exception for more details. However, because of the async nature of the task, this test failure may look like a false positive. Perhaps you already have a Guard Clause in place, but inside the Task or inside a method marked with the 'async' keyword (if you're using C#); if this is the case, the Guard Clause is dormant, and will first be triggered when a client accesses the Result of the Task. This doesn't adhere to the Fail Fast principle, so should be addressed.
See https://github.com/AutoFixture/AutoFixture/issues/268 for more details.

This addresses #268.