[BUGFIX 3.28] Improve assert in default store

[mixonic]

Mirrors #19858, but for Ember 3.28.

When a route has a dynamic segment, several implicit behaviors may be provided by the Ember framework or by the framework in conjunction with a library (commonly Ember Data).

• If the route class exists and has a model( hook implemented, then that logic is the explicit implementation for data loading on that route. In the far future (say, Ember 5), we want explicit data loading to be the only way data can be loaded.
• However, today there are several implicit data loading behaviors. First, the user may explicitly inject a store service into the route. If a model hook is not implemented, the find method on the store will be called. This is, as of this writing, valid in Ember 3 & 4 and not deprecated.
• If a model hook is not implemented and if an explicit injection is not present, a container/owner-based implicit type or factor injection may have added the store. This if valid in Ember 3, but deprecated in Ember 3.28.7 (see [BUGFIX 3.28] Improve implicit injections deprecation for routes #19854), and does not occur in Ember 4 since implicit injections are no longer supported.
• If a model hook is not implemented, and if neither an explicit or implicit injection of store is made, then Ember provides a minimal "default store" implementation. This looks up a model matching the route name, and calls a find method on that model. If there is no model, an assertion is thrown. If there a model but it has no find, an assertion is thrown.

This patch updates those assertions (which are very old) to better coach developers toward the APIs we prefer in modern development. These are:

• If you have a route with dynamic segments, you should implement a model hook on the route.
• Alternatively, and as a transition step for existing code, you may explicitly inject a store on the route to preserve behavior in an application which was relying on implicit injections.

There are a lot of edge cases to consider in these copy changes. Many apps use Ember Data. Some apps use different data layers which also provide a store service. Some applications implement their own store and may implicitly inject it. Some use no store at all!

Many apps use dynamic segments. Developers may not realize that if a model hook (or route class) has not been defined in Ember and Ember Data 3.28, the store service is being relied upon and causes these assertions to never be run.

In Ember 3.28 accessing the implicit injection of store (and indeed all implicit injection consumption) is deprecated, so existing code should be upgraded to explicit injections and these assertions should never come into play.

In Ember 4.0 developers who author:

• A new dynamic segment route.
• ...with no route class or no model hook.
• ...and no explicit service injection.
• ...but who also have model type factories (likely through Ember
  Data convention)

Would see the second of the updated assertions here. This patch updates the message for 3.28 since it is plausible some non-conventional apps could be seeing this message in 3.x, especially as they upgrade to Ember 4.0.