Improve error information on a test operation

[kitsonk]

Enhancement

When attempting to apply an undo operation that is failing for some reason because the test operation does not pass, currently all you get is:

Error: Test operation failure. Unable to apply any operations.

An error message the provides more context would be more useful, like:

Error: Test operation failed.
    Expected "somevalue" for at path "/foo/bar/baz" but got "othervalue"

Potentially looking AssertionError and potentially moving that to core. The other advantage is it will utilise V8's diffing functionality when thrown, improving the information available in Chrome and Node.