add better error for passing config with no steps
add better error for passing config with no steps

Summary: in response to https://github.com/facebook/openr/issues/23

Reviewed By: snarkmaster

Differential Revision: D7342371

fbshipit-source-id: 15737f79900722ce016494f994932aa26e5acad5

facebook/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,rklabs/folly