Improve playbook role name error reporting

Previously, the documentation had a ``{ name: apache, http_port: 8080 }`` example in it, but if one were to implement a ``roles`` dictionary in that manner, it would error out with the message:
> ERROR: expected a role name in dictionary: {'name': 'java'}

Aside from fixing the documentation, this PR updates the message so that it now dies like this:
> ERROR: expected a role name in key 'role' of dictionary: {'name': 'java'}

I feel this is clearer.