Throw exception when protected resource deligate is nil.

[jpittis]

Because we can now disable bulkheads or circuit_breakers for a protected_resource, we should raise an exception trying to access methods of a nil delegate.

For example, with bulkhead: false, resource.count should raise a BulkheadDisabledError rather than a NoMethodError kinda thing.

We should also add something to the docs that recommends using resource.bulkhead.count over resource.count.

cc @sirupsen @csfrancis