Make MockFactory.VerifyMocks do what it says it does

[stakx]

The XML documentation comment for MockFactory.VerifyMocks(Action<Mock> verifyAction) states the purpose of that method as follows:

Invokes verifyAction for each mock in [the repository], and accumulates the resulting verification exceptions that might be thrown from the action.

However this is not what the method has been doing up until now. It hasn't accumulated any verification errors, but only those caused by unmatched setups (i.e. those exceptions thrown by Verify and VerifyAll).

This PR makes the VerifyMocks method adhere to its own stated purpose by cleaning up MockException aggregation a little. This also allows us to reuse VerifyMocks in MockFactory.VerifyNoOtherCalls.

[stakx]

/cc @BlythMeister — you were right about this. You proposed very similar changes a short while ago, which I turned down at the time. I'd like to apologize for that. I've only just realised now that I am going to need clean MockException aggregation anyway for an upcoming refactoring.

[BlythMeister]

@stakx no need to apologize! Just glad that my change made you look at this code 👍