Have human readable error for no config file found

[jcesario]

When no config file is able to be read, agent fails with

if len(config.Config.AgentsServer) == 0 { log.Fatal("AgentsServer unconfigured. Please set to the HTTP address orchestrator serves agents (port is by default 3001)") }

If no config file can be read orch-agent should fail gracefully with a more usable error code.