Distinguish between permanent and temporary Onionoo errors
As of now, Onionoo has a "maintenance mode" that I'm planning to use very rarely. But sometimes it's necessary to shut down Onionoo to support new features or fix bugs. During this time, Onionoo responds to all requests with a 503 Service Unavailable status code. Atlas should respect this code and display a different warning than:
"Backend error! The backend server replied with an error to your query. This probably means that you did not properly format your query. If your query was properly formatted it may mean that there is an issue with your browser/add-ons. Please report which browser/addons/etc. you're using to the bug tracker."
How about this warning?
"Backend temporarily unavailable! The backend server is temporarily unavailable. If this issue persists for more than a few hours, please report it using the bug tracker."