Remove catch (...) from handleIncomingMsg

This will help us get stack dumps on all unhandled exceptions on Windows,
not just those, which occur on non-query threads.

Original description:

jsCore/where4.js fails occasionally on Windows, when run with small oplog.

The cause of the failure is emergency server shutdown due to unhandled exception, which is neither of assertion, MongoDB exception or STL exception.

Steps to Reproduce: Happens randomly, no specific steps to reproduce other than running the jsCore suite with small oplog.