Make error handling requirements explicit

[ansor4]

Currently we have:

If a request is unsuccessful, output a simple error message via console.log() or puts and recover.

Change this to:

If a request is unsuccessful, output an error message (for example, "Server returned 404") via console.log() or puts, then recover.

Also, add test cases to client_spec.rb and client.test.js to test for error handling.

What we currently have can be confusing to candidates as to how defensive they need to be.