iqdbs: raise exception if not configured.

app/controllers/iqdb_queries_controller.rb

@@ -4,8 +4,7 @@ class IqdbQueriesController < ApplicationController
 
   def create
     if !Danbooru.config.iqdbs_server
-      render :nothing => true
-      return
+      raise NotImplementedError.new("the IQDBs service isn't configured. Similarity searches are not available.")
     end
 
     if params[:url]
@@ -32,4 +31,4 @@ protected
     @results = @download.matches
     render :layout => false, :action => "create_by_post"
   end
-end
+end