Prepare CLI for extension sunsetting

Created by: philipp-spiess

Part of sourcegraph/sourcegraph#39044 Part of sourcegraph/sourcegraph#39043

While auditing the CLI functionality and requirements for the upcoming extensions deprecation, I noticed two areas that are currently not working well:

• The sg extensions command doesn't explain the upcoming deprecation yet. I've changed it to show the same text as sourcegraph.com cc @jjinnii @ryankscott @muratsu
• The sg extensions list command did not forward the GraphQL error probably. This happens because the endpoint does not use the normal GraphQL error pattern for errors. See this for more context.

All other extension endpoints work as expected and I made sure they forward an error message sent on the backend (I'll follow up with the backend changes to gate out the APIs in a separate commit [lol obviously because this is a different repo haha]).

Test plan