Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
So what was confusing me is that I designed `RSpec::Mocks::Constant.original` to not raise errors but to instead return an object with rich details about the constant (such as if it was ever defined).  The case of a totally invalid const name (like an anonymous class's `inspect`) wasn't one I considered, but I consider the `NameError` in that case to be a bug in `RSpec::Mocks::Constant.original`.  I put together a PR to fix that in #833.  You should be able to remove this `rescue` with that in place.