Follow-up to #1773, which meant to add more useful query errors but
did not actually do so. Since that patch, any error other than
interrupt/cancel/timeout was reported as `{"error":"Unknown exception"}`.
With this patch, the error fields are:
- error, one of the specific strings "Query interrupted", "Query timeout",
"Query cancelled", or "Unknown exception" (same behavior as before).
- errorMessage, the message of the topmost non-QueryInterruptedException
in the causality chain.
- errorClass, the class of the topmost non-QueryInterruptedException
in the causality chain.
- host, the host that failed the query.