Add error checking for hostPort range

This fix catches the case where there is a single container port
and a dynamic host port and will fail out gracefully
Example docker-compose.yml snippet:
    port:
        ports:
            - "8091-8093:8091"
            - "80:8080"

Signed-off-by: Tony Abboud <tdabboud@hotmail.com>

compose/convert/service.go

@@ -301,9 +301,11 @@ func convertEndpointSpec(source []string) (*swarm.EndpointSpec, error) {
 	}
 
 	for port := range ports {
-		portConfigs = append(
-			portConfigs,
-			opts.ConvertPortToPortConfig(port, portBindings)...)
+		portConfig, err := opts.ConvertPortToPortConfig(port, portBindings)
+		if err != nil {
+			return nil, err
+		}
+		portConfigs = append(portConfigs, portConfig...)
 	}
 
 	return &swarm.EndpointSpec{Ports: portConfigs}, nil