Reject synchronous batch callback failures

[simPod]

Summary

• Convert synchronous batch callback throws into rejected load promises.
• Reject every queued load through the existing whole-batch failure path.
• Cache promises before immediate dispatch so failed non-batched loads can retry.

Motivation

A callback that throws before returning a promise escapes dispatch and can leave queued promises unresolved or cache a failed immediate load. The current master Throwable contract handles both Exception and Error consistently.