UX: error messages include next-step hints

Error messages should tell the user what to do next, not just what
went wrong. Audit current error paths and update messages where the
remediation isn't obvious.

## Definition of Done

- [ ] Error message audit complete
- [ ] Updated messages land with next-step guidance
(e.g. "Run `func init` to create a new project.")
- [ ] Style guidance added to contributor docs

Start by locating the current CLI error paths and contributor documentation, then inventory messages whose remediation is not obvious. Done means the audit is complete, updated messages include next-step guidance, and the contributor docs describe the message style.

Written by the indexing model from the issue text.