Fix #1377: Remove flakyness of JSEnv test

The timeout tests rely on timing which is inherently unrealiable. By
ensuring large enough intervals between (expected) events, we
hopefully can eliminate flakyness.

This commit increases intervals from the order of 1ms to 10ms.
Fix #1375: Don't propagate all Rhino load exceptions to JS
Merge pull request #1379 from gzm0/rhino-fixes

Rhino fixes