lib/registry: `getModel` throws, add `findModel`

Rename `loopback.getModel` to `loopback.findModel`.

Implement `loopback.getModel` as a wrapper around `findModel` that
throws an error when the model as not found.
Merge pull request #331 from strongloop/make-get-model-throw-when-not-found

[2.0] Make getModel throw when not found