Unexpected change of behaviour in MatchError() matcher

[blgm]

In Gomega 1.7.1 the following used to pass:

type CustomError struct {}

func (c CustomError) Error() string {
	return "an error"
}

c := CustomError{}
Expect(&c).To(MatchError(&c))

With Gomega 1.8.0, the test above fails with:

      Expected
          <*matchers_test.CustomError | 0x19c1858>: {}
      to match error
          <*matchers_test.CustomError | 0x19c1858>: {}

The reason is that when the extensions for testing wrapped errors was added in #359, we assumed that we would always pass error values rather than pointers. This is not always the case, so some consumers of Gomega may be broken.