scripts: add error code argument to sendClientError helper

And update all users accordingly. This will allow the following commit
to use the helper with a different error code.

src/scripts/lib/linking-utils.lua

@@ -314,7 +314,7 @@ function lutils.haveAvailableRoutes (si_props)
   return false
 end
 
-function lutils.sendClientError (event, node, message)
+function lutils.sendClientError (event, node, code, message)
   local source = event:get_source ()
   local client_id = node.properties ["client.id"]
   if client_id then
@@ -323,7 +323,7 @@ function lutils.sendClientError (event, node, message)
         Constraint { "bound-id", "=", client_id, type = "gobject" }
     }
     if client then
-      client:send_error (node ["bound-id"], -2, message)
+      client:send_error (node ["bound-id"], code, message)
     end
   end
 end

src/scripts/linking/find-defined-target.lua

@@ -118,7 +118,7 @@ SimpleEventHook {
       local linger = cutils.parseBool (si_props ["node.linger"])
       if not linger then
         local node = si:get_associated_proxy ("node")
-        lutils.sendClientError (event, node, "defined target not found")
+        lutils.sendClientError (event, node, -2, "defined target not found")
         node:request_destroy ()
         log:info(si, "... destroyed node as defined target was not found")
       else

src/scripts/linking/find-filter-target.lua

@@ -79,7 +79,7 @@ SimpleEventHook {
       local linger = cutils.parseBool (si_props ["node.linger"])
       if not linger then
         local node = si:get_associated_proxy ("node")
-        lutils.sendClientError (event, node, "smart filter defined target not found")
+        lutils.sendClientError (event, node, -2, "smart filter defined target not found")
         node:request_destroy ()
         log:info(si, "... destroyed node as smart filter defined target was not found")
       else

src/scripts/linking/prepare-link.lua

@@ -111,7 +111,7 @@ SimpleEventHook {
         return
       end
 
-      lutils.sendClientError (event, node,
+      lutils.sendClientError (event, node, -2,
           reconnect and "no target node available" or "target not found")
 
       if not reconnect then