title: HTTP Status Code for External Dependency Error
body: <p>What is the correct HTTP status code to return when a server is having issues communicating with an external API?</p>
  
  <p>Say a client sends a valid request to my server A, A then queries server B's API in order to do something. However B's API is currently throwing 500's or is somehow unreachable, what status code should A return to the client? A 5* error doesn't seem correct because server A is functioning as it should, and a 4* error doesn't seem correct because the client is sending a valid request to A.</p>