simple_engine/stores: replace panic!/unwrap/expect with Result propagation

[milindsrivastava1997]

Follow-up to #100 (closed without landing this half).

92 occurrences of panic!/unwrap()/expect(...) across simple_engine/ (21) and stores/ (71, promsketch_store + simple_map_store). Most of these functions already return Result<_, String>, so a panic deep in a store call takes down the whole query engine instead of surfacing as a query error.

Fix: convert to Result propagation — .ok_or_else(|| format!(...))? / .map_err(...)? instead of .expect(...)/panic!, threaded up through existing Result return types.