Don't raise NameErrors in `be` matcher

@myronmarston I realise you [suggested](https://github.com/rspec/rspec-expectations/issues/470#issuecomment-38987850) this was a bad idea, but it's consistent with the current implementation @JonRowe introduced in https://github.com/rspec/rspec-expectations/commit/dc32c7b07a1860b1c076eb45d7485b7d6b453d1c. If you want, I can refactor both `raise`s to work the same way as the exists matcher.