[Minor] Fix DNS lookup errors

src/plugins/lua/multimap.lua

       if rt == 'ip' then
         match_rule(rule, ip)
       else
-        local cb = function (_, _, results, err)
-          if err then
-            rspamd_logger.errx(task, 'DNS lookup failed: %s', err)
+        local cb = function (_, to_resolve, results, err)
+          if err and (err ~= 'requested record is not found' and err ~= 'no records with this name') then
+            rspamd_logger.errx(task, 'error looking up %s: %s', to_resolve, err)
           end
           if results then
             task:insert_result(rule['symbol'], 1, rule['map'])

src/plugins/lua/once_received.lua

   local recvh = task:get_received_headers()
 
   local function recv_dns_cb(_, to_resolve, results, err)
-    if err then
-      rspamd_logger.errx(task, 'DNS lookup error: %s', err)
+    if err and (err ~= 'requested record is not found' and err ~= 'no records with this name') then
+      rspamd_logger.errx(task, 'error looking up %s: %s', to_resolve, err)
     end
     task:inc_dns_req()
 

src/plugins/lua/rbl.lua

 local function rbl_cb (task)
   local function gen_rbl_callback(rule)
     return function (_, to_resolve, results, err)
-      if err then
-        rspamd_logger.errx(task, 'DNS lookup error: %s', err)
+      if err and (err ~= 'requested record is not found' and err ~= 'no records with this name') then
+        rspamd_logger.errx(task, 'error looking up %s: %s', to_resolve, err)
       end
       if not results then return end