Print errors that end an fx command in plain words

Summary

• fx ask prints an error that ends it in plain words instead of its name: fx: unreadable reply for a stream event too deep to read, where it printed fx: InvalidGatewaySseEvent, and fx: session not found for an unknown --resume session.
• Other fx commands that end on an error with no message of their own print it in plain words too. The error name still goes to the trace log, and fx ask --json still reports it in error.
• An unreadable or oversized stream event from the Codex or Grok provider reads as "unreadable reply" or "reply too large to read", as one from the Gateway does.