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.

src/helper/command.c

@@ -314,9 +314,8 @@ static int register_command_handler(struct command *c)
 	}
 
 	/* we now need to add an overrideable proc */
-	const char *override_name = alloc_printf("proc %s {args} {"
-			"if {[catch {eval ocd_%s $args}] == 0} "
-			"{return \"\"} else {return -code error}}",
+	const char *override_name = alloc_printf(
+			"proc %s {args} {eval ocd_bouncer %s $args}",
 			full_name, full_name);
 	if (NULL == override_name)
 		goto free_full_name;