Name the cause when fx can't connect to the model catalog

Summary

• When fx can't connect to the model catalog, including a catalog request that times out, it prints one line that says so and asks the user to check their network connection or firewall.
• That line also says what fx holds back until the catalog loads: earlier images, images from tools, fast mode, or a combination of them.
• For connection failures, the line replaces the separate "Earlier images aren't reaching the model right now..." and "Fast mode is unavailable for this model right now..." lines, and it still appears once per turn.
• When an image attached to the turn can't be sent because fx couldn't connect to the catalog, fx first prints a line that names the failed connection, then the existing "Unable to verify image support..." error.
• fx models says it couldn't connect to AI Gateway and asks the user to check their network connection or firewall, in place of "could not list models: Unavailable". The JSON code stays TransportFailure.
• fx provider and fx login say they couldn't connect to load the target model catalog, in place of "could not load the target model catalog (transport)".
• The messages name AI Gateway for the gateway provider and "the model provider" for other providers.
• A catalog that answers with an error, such as a 5xx response, keeps the existing messages.