Current behavior
Currently the various libunbound functions report failures via UB_* constants, which unbound.h doesn’t expose. It would be helpful for error handling to have those constants in 3rd-party applications.

Describe the desired feature
Put enum ub_ctx_err into unbound.h so 3rd party apps can access it.

Potential use-case
If I want to translate the errors or otherwise recover from them it’s good to have error codes instead. And it’s ideal not to have to copy-paste them from context.h into my own code.

I’m happy to submit a PR to do this, if it’s an agreeable proposal.

Thank you!