Model search API has different behavior depending on if system index is created or not

[jmazanec15]

Description

Currently, when running the model search api, if the model index does not exist, it will throw an exception. If it does exist, it will return empty hits. The problem with this is that it leaks implementation details about the model system to the user. A user in general should not need to know about the backing system index in order to use the functionality.

Expected Behavior

If there are no models matching a query, return a consistent result, regardless of whether the backing index is created or not.