Add matcher reflecting errors.Is behavior

[Smerom]

The current MatchError matcher considers errors to be a match that errors.Is does not, and so cannot be used in a test to ensure that upstream error handling can use errors.Is to for error specific handling. In order to not break any existing tests, I propose adding the following stricter error matcher that considers two errors a match if and only if they are both non-nil, and errors.Is(actual, expected) returns true.

As an example of current behavior:

Expect(errors.New("some error")).To(MatchError(errors.New("some error")))

will pass, however:

errors.Is(errors.New("some error"), errors.New("some error"))

will return false.

Related to issue: #876