Show error for unsupported references to operations or models

[margarita-v]

If swagger has features which are not supported yet:

paths:
  /auth/login:
    $ref: "./auth/auth_login.yml"

schemas:
    $ref: "./definitions.yml"

than surfGen lint command will be successful anyway (probably because linter has an empty list of operations and finishes successfully). But surfGen generate command fails with [1] 90705 trace trap error. It would be great if SurfGen showed the detailed error message for this unsupported case, because currently it is not obvious and seems to be wrong