fixes issue #831 and issue #771 by checking for unset active machine name

Signed-off-by: Ken Pepple <ken@solinea.com>

commands.go

@@ -760,6 +760,10 @@ func cmdSsh(c *cli.Context) {
 			log.Fatalf("unable to get active host: %v", err)
 		}
 
+		if host == nil {
+			log.Fatalf("There is no active host. Please set it with %s active <machine name>.", c.App.Name)
+		}
+
 		name = host.Name
 	}
 
@@ -768,6 +772,15 @@ func cmdSsh(c *cli.Context) {
 		log.Fatal(err)
 	}
 
+	_, err = host.GetURL()
+	if err != nil {
+		if err == drivers.ErrHostIsNotRunning {
+			log.Fatalf("%s is not running. Please start this with docker-machine start %s", host.Name, host.Name)
+		} else {
+			log.Fatalf("Unexpected error getting machine url: %s", err)
+		}
+	}
+
 	if len(c.Args()) <= 1 {
 		sshCmd, err = host.GetSSHCommand()
 	} else {
@@ -880,6 +893,9 @@ func runActionWithContext(actionName string, c *cli.Context) error {
 		if err != nil {
 			log.Fatalf("Unable to get active host: %v", err)
 		}
+		if activeHost == nil {
+			log.Fatalf("There is no active host. Please set it with %s active <machine name>.", c.App.Name)
+		}
 		machines = []*libmachine.Host{activeHost}
 	}