URL-not-found message went into i18n JSON resource

Yet, the URL-not-found message is not yet fully internationalized
since its usage is hardcoded to English.

static/i18n/en.json

@@ -5,4 +5,5 @@
 	},
 	"name":"English",
 	"suggest-full-text-search": "containing '{{{SEARCH_TERMS}}}'..."
+	, "url-not-found" : "The requested URL \"{{url}}\" was not found on this server."
 }

static/i18n/qqq.json

@@ -6,4 +6,5 @@
 	},
 	"name": "Current language to which the string is being translated to.",
 	"suggest-full-text-search": "Text appearing in the suggestion list that, when selected, runs a full text search instead of the title search"
+	, "url-not-found" : "Error text about wrong URL for an HTTP 404 error"
 }