Throw error when resolving references if reference data is not found

[kmudrick]

No description provided.

[kmudrick]

This will cause us to see:

Error: Could not find parameters/someBogusParameter in /path/to/src/foo/specs/bar.yaml
    at /path/to/src/foo/node_modules/oas-nodegen/lib/utilities.js:102:17
   ...

Whereas the current behavior is to pass along undefined and later on you get hit with a Cannot read property 'name' of undefined error message and no context / hint as to where it went wrong or what was being processed.

[pkedy]

Thanks for the PR!