Fix silent error-swallowing in Firebolt scripts; drop bogus t3a.small results

[thevar1able]

Resolves #945

Summary

firebolt/query and firebolt/load (and their -parquet/-parquet-partitioned
copies) checked for failure with jq -e '.errors' ..., but jq -e also exits
non-zero on malformed or empty input — which the old code treated identically
to "no .errors key present," i.e. success. A crashed/overloaded engine that
comes back with an empty 200 body or a non-JSON body could silently be
recorded as a fast, error-free query with no timing on stderr, or (for load)
surface only as an opaque curl: (22) ... error 500 with the actual reason
discarded.

• query: now checks HTTP status, non-empty body, and JSON-validity
  explicitly before checking .errors, and validates .statistics.elapsed
  is actually numeric before declaring success.
• load: same idea via a run_sql helper — prints the real response body
  on any non-200/invalid-JSON/.errors response instead of swallowing it.

Validated that both cases fail to load dataset on t3a.small:

Load time: 0.185
bench: data-size after load is '211108' (<5 GB)
bench: ClickBench's hits dataset doesn't fit in <5 GB on any
bench: system in the catalog; treating this as a partial load
bench: (likely an OOM kill mid-COPY).
Disk usage after: 35281371136
System: firebolt-parquet
Machine: t3a.small
Total time: 233

Load time: 0.986
bench: data-size after load is '170112' (<5 GB)
bench: ClickBench's hits dataset doesn't fit in <5 GB on any
bench: system in the catalog; treating this as a partial load
bench: (likely an OOM kill mid-COPY).
Disk usage after: 35239878656
System: firebolt-parquet-partitioned
Machine: t3a.small
Total time: 144