fix: better error message when pubsub is not enabled

[alanshaw]

If pubsub is disabled there's no pubsub property on the libp2p node and we get the error "Cannot read property 'subscribe' of undefined". This is really confusing. This PR detects disabled pubsub and throws an error with a better message.

[vasco-santos]

Changes LGTM!

I have been thinking that we should have a getter for pubsub and dht exposed from libp2p, and the error in that case would come from there.

In my opinion, we can move with this solution and if you and @jacobheun agree, I can implement the getters with the errors in js-libp2p and update js-ipfs later.

[jacobheun]

I'd like to take advantage of the await/async migration and revisit libp2p's api to help avoid applications needing to do these kind of checks, it's annoying to need to check every method like this.

I think the libp2p public apis should always be invokable and error appropriately. libp2p core is really just exposing services that have been bundled with it. We should ensure those api calls are simple and informative when they do error. I'd expect to be able to just call libp2p.pubsub.ls, etc, and have it return this type of error.

This is fine as a interim solution, but libp2p should work to make sure this commit is revertible in the near future.

I'll get with @vasco-santos and we can talk through some options for improving the API and create a proposal for feedback for the migration.

[alanshaw]

I think what @vasco-santos suggested is an interesting idea, but it would need to wait for the async/await refactor, otherwise we'd still need code to deal with the throw in js-ipfs and return it in the callback.

libp2p.pubsub can be a getter that throws if pubsub is not enabled. When we have an async/await API, an app using libp2p will (likely) have a try/catch around the call anyway:

class Lib2p {
  /* ... */
  get pubsub () {
    if (!this.options.pubsub.enabled) throw new Error('pubsub not enabled')
    return this._pubsub
  }
}

const libp2p = new Libp2p

try {
  await libp2p.pubsub.subscribe(/*...*/)
} catch (err) {
  console.log(err.message) // pubsub is not enabled
}