Change hostname-in-use error to be more usable

[paulproteus]

Context:

• A user had to recover a domain name.
• They stared at the message for a minute or so, until they figured out that typing help was the right thing to do.
• The current message is:

This hostname is already in use. Type help if you need to recover access, or pick a new one.

Proposal:

Use this as the new hostname-in-use error message:

This hostname is already in use. If you're the person who registered it, type "help" to start the domain recovery process. Or you can type a new hostname to use.