V1 [UX3] Errors and troubleshooting

[austin-barrington]

Context

Known sharp edges (e.g. libchdb / startup failures mentioned in README) should be discoverable and actionable.

Deliverables

• Expand docs/user-guide/troubleshooting.md with symptoms → cause → fix for top support issues.
• Cross-link from error messages in server where safe (or stable error codes later).

References

docs/user-guide/troubleshooting.md; README.md (bad_function_call note)