Thorough aggregation of verification errors & more complete error messages

[stakx]

Verification using mock.Verify[All] has always been performed in a "recursive" manner (i.e. inner mocks were verified along with the outer / owning mock). However, in the case of failure, verification would often stop and only describe the first error in the message.

This PR makes verification more thorough by aggregating verification errors at all levels:

• Mock repositories aggregate errors of all their mocks. (This was done already.)
• Mocks aggregate errors of all setups. (This is new.)
• Setups can forward errors from an inner mock. (This is also new.)

For example, given this code:

var mock = new Mock<IOuter>();
mock.Setup(m => m.Inner.Method());
mock.Setup(m => m.AnotherMethod());
mock.VerifyAll();

you might now see such error message:

Mock<IOuter:00000001>:
This mock failed verification due to the following:

   IOuter mock => mock.Inner:

      Mock<IInner:00000001>:
      This mock failed verification due to the following:

         IOuter m => m.Inner.Method():
         This setup was not matched.

   IOuter m => m.AnotherMethod():
   This setup was not matched.

instead of the previous:

The following setups on mock 'Mock<IOuter:00000001>' were not matched:
IOuter mock => mock.Inner
IOuter m => m.AnotherMethod()

The trade-off here is obviously brevity vs. completeness. The latter probably makes more sense in an error message (IMO).