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.