added lang strings for not having enough rep to downvote

public/language/en_GB/error.json

@@ -72,5 +72,7 @@
 
 	"signature-too-long" : "Signature can't be longer than %1 characters!",
 
-	"cant-chat-with-yourself": "You can't chat with yourself!"
-}
+	"cant-chat-with-yourself": "You can't chat with yourself!",
+
+	"not-enough-reputation-to-downvote": "You do not have enough reputation to downvote this post"
+}

public/language/en_US/error.json

@@ -50,5 +50,6 @@
     "uploads-are-disabled": "Uploads are disabled",
     "upload-error": "Upload Error : %1",
     "signature-too-long": "Signature can't be longer than %1 characters!",
-    "cant-chat-with-yourself": "You can't chat with yourself!"
+    "cant-chat-with-yourself": "You can't chat with yourself!",
+    "not-enough-reputation-to-downvote": "You do not have enough reputation to downvote this post"
 }