improve command handler wrapper script

improve command handler wrapper script

Adds 'ocd_bouncer' in startup.tcl that is called as a helper for
all command handlers, shrinking the embedded C wrapper to a mere stub.

Jim handlers are called directly, simple handlers get called with the
wrapper to capture and discard their output on error, and placeholders
call help directly (though the unknown handler still does this too).
It attempts to improve the quality of the error messages as well.