Make ThemeProvider error straightforward

[lekterable]

Super small change, just for the sake of keeping errors dead simple and straightforward.
I think that theres a chance that it might save someone a bit of confusion.

[quantizor]

This is unfortunately a breaking change. We can't accept this for v4 or v5, but can consider it for v6. nevermind, I see this is just making a more-clear error. Ok cool.

Can you do this for canary instead?

[lekterable]

that should do it, let me know what you think @probablyup

does the sandbox work on the canary btw? when I'm trying to access the page after yarn start and installing the modules I get:

I've tried on the clean branch as well so It's not because of my changes.

[quantizor]

@lekterable ah yeah just run build first (from root I think) so it generates the file locally

[lekterable]

yup, that did it, thanks @probablyup ! I've tried it in sandbox on canary and works as expected so It's ready to merge! :)

[quantizor]

Thank you!