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;

src/helper/startup.tcl

@@ -10,6 +10,30 @@ proc exit {} {
 	ocd_throw exit
 }
 
+# All commands are registered with an 'ocd_' prefix, while the "real"
+# command is a wrapper that calls this function.  Its primary purpose is
+# to discard 'handler' command output,
+proc ocd_bouncer {name args} {
+	set cmd [format "ocd_%s" $name]
+	set type [eval command type $cmd $args]
+	if {$type == "native"} {
+		return [eval $cmd $args]
+	} else {if {$type == "simple"} {
+		if {[catch {eval $cmd $args}] == 0} {
+			return ""
+		} else {
+			set errmsg "Command handler execution failed"
+		}
+	} else {if {$type == "group"} {
+		catch {eval help $name $args}
+		set errmsg [format "%s: command requires more arguments" \
+			[concat $name " " $args]]
+	} else {
+		set errmsg [format "Unknown command type: %s" $type]
+	}}}
+	return -code error $errmsg
+}
+
 # Try flipping / and \ to find file if the filename does not
 # match the precise spelling
 proc find {filename} {