A bug surfaced where first time evaluation of a model fails due to the
Model constructor throwing if the model does not exist.

Looking deeper, we see that most calls to get_model expect a possible
None response and check at the call site. Unfortunately we get the same
WebserviceException class for a model not being found as we do a REST
error or similar.

This change is a stopgap mitigation to restore compatibility with the
existing callers, and compromises by allowing the model version
dependent behavior to continue passing on exceptions.

In a future follow up we should settle on a convention and allow version
checks to propagate failure while still giving the possibility for
handling a service exception in the caller.