reportbooru: raise exception if not configured.

app/logical/popular_search_service.rb

@@ -2,7 +2,15 @@
 class PopularSearchService
   attr_reader :date, :scale
 
+  def self.enabled?
+    Danbooru.config.reportbooru_server.present?
+  end
+
   def initialize(date, scale)
+    if !PopularSearchService.enabled?
+      raise NotImplementedError.new("the Reportbooru service isn't configured. Popular searches are not available.")
+    end
+
     @date = date
     @scale = scale
   end