Improve error message when no registrations are available

[ibraheem-abe]

We need to be more verbose and informative about the feedback.

btcli subnets register

https://discord.com/channels/799672011265015819/1364771192606556210

[Mobile-Crest]

@ibraheem-abe
I'd like to work on this issue
can you please assign it to me?

[ibraheem-abe]

Yes you can work on this

[Mobile-Crest]

this issue may be closed now?

[Mobile-Crest]

@ibraheem-abe @thewhaleking
can you please close this issue? thanks

[codomposer]

is this issue resolved?