assert.NotErrorAs: new assertion

[titusjaka]

Summary

Add NotErrorAs methods to check that none of the errors in err's chain matches target.

Changes

• add NotErrorAs and NotErrorAsf assertions to assert and require packages

Motivation

Sometimes, it's essential to check that error does not match an exact type.

func TestExample(t *testing.T) {
	err := errors.New("test error")

	var notFoundErr ErrNotFound
	assert.NotErrorAs(t, err, &notFoundErr)

	err = WrapNotFoundError(err)
	assert.ErrorAs(t, err, &notFoundErr)
}

Related issues

Closes #1066

[johanneswuerbach]

I was just searching for this function, to discover it is missing.

Maybe @boyan-soubachov could have a look? :-)

[Antonboom]

+1 for this PR.
and take a look to #1129