Needs better error messages

[MRAAGH]

When starting out, I kept getting "this configuration does not work" in octocmd. I spent hours debugging, I noticed that accessing http://localhost/api/files?apikey=12345678909876543123456789 worked fine. Eventually I found out that my printer was not currently connected to octoprint, and that's why octocmd was not accepting my credentials. Better error messages could prevent such situations and lower the entrance barrier! Or, if not error messages, we should at least have some instructions somewhere which mention these things!