better error message for extension point

[aspeddro]

• Remove the bs. prefix
• Add extension name to the message

[aspeddro]

I'm still going to add tests.

[cristianoc]

This looks great.
I think there's no need to add automatic tests, as long as each case is tested at each once manually. Unless you think this might regress in future.

[cristianoc]

Ready to merge after a tiny change.

[cristianoc]

Thanks!