Show error message when using invalid plugin or theme slugs

[annezazu]

In a moment of moving too fast, I tried loading a Playground with Yoast installed to test something with metaboxes only to get an error: https://playground.wordpress.net/?plugin=yoast The problem? I had the login plugin slug! I should use https://playground.wordpress.net/?plugin=wordpress-seo. However, in loading https://playground.wordpress.net/?plugin=yoast, instead of being met with a helpful error message cluing me in to my mistake, I instead ran into the following errors:

We should offer a more helpful error message when folks get this wrong and load something invalid!

[adamziel]

@bgrgicak is it the same issue as we've discussed recently? Should I assign you? Or is it a different problem?

[bgrgicak]

This is part of what we discussed. I can work on it.

[bgrgicak]

For this feature, I wanted to modify the Blueprints installTheme and installPlugin steps and add a slug check.
If the slug check fails, it could throw a custom error object that can be detected by the Playground website, which can use it to trigger a custom modal with an error message.

Before we do any work on this, let's check Blueprints v2 to see if it already does a slug check, but no matter if it does or not, we will still need to create an improved error modal.