make actionable error messaging when DHT not enabled

[jacobheun]

If no content/peer router is running, such as the DHT, and a user attempts to make a content or peer routing request the error is not particularly helpful as detailed at https://github.com/libp2p/js-libp2p/issues/314#issuecomment-458802548.

For errors like this, where the system needs to be added/started via configuration, we should add an actionable message to the error to allow the user to better fix the problem. In the error linked above, this could be a link to the DHT configuration section of the readme.

cc: @vasco-santos

[vasco-santos]

Having a reference to the README seems a cool idea!

[maschad]

Closing as this is no longer relevant, we now return a more informative error