Friendly warning for missing models

[jasonmccreary]

Synopsis:

It's possible if a user runs blueprint:build for an existing app (or a draft.yaml file with references the User model) it's likely they may receive a nasty error.

This is often mitigated by running blueprint:trace. Yet there is no indication to do so. In the end, this is a poor out-of-the-box experience.

Expected Behavior:

Given the following draft.yaml file, it would be better if an error message were thrown letting the user know they have an "unknown model" and to either define it or run blueprint:trace if it exists already.

controllers:
  User:
    resource: api

A centralized place to throw this error might be modelForContext. However, sometimes Blueprint handles a missing models. So these will likely need to be reviewed, but maybe a second throw parameter could be added.

[devmsh]

While modelForContext invoked in the generation phase, I think we can check the missing models earlier after the analysis phase even before generate any valid controllers and throw the exception in the middle of the process.

What about adding extra validation step inside Blueprint::analyze where we have the full Tree and it may be extended in the future to make extra validations?

https://github.com/laravel-shift/blueprint/blob/be5f383484c0a07e0df32e57d8d03198ab57c4a5/src/Blueprint.php#L59-L71