Improve error messaging when Playground Block is used on non-ssl site

[ndiego]

I ran into this when testing out the Playground Block in a local environment that was not https. The error message you see is the following. It would be great if the error mentioned that Playground needs to be run with https if the user is attempting to use the block on http.

Generic error

[adamziel]

I'm moving this issue to the Playground repo as the error message comes from core