feat: ensure network exists before attempting to attach it to a container

This will give better feedback to developers than a potentially generic error.

plugins/network/triggers.go

@@ -174,6 +174,15 @@ func TriggerPostContainerCreate(containerType string, containerID string, appNam
 		return
 	}
 
+	exists, err := networkExists(networkName)
+	if err != nil {
+		common.LogFail(err.Error())
+	}
+
+	if !exists {
+		common.LogFail(fmt.Sprintf("Network %v does not exist", networkName))
+	}
+
 	attachAppToNetwork(containerID, networkName, appName, phase, processType)
 }
 
@@ -208,6 +217,15 @@ func TriggerCorePostDeploy(appName string) {
 		common.LogFail(err.Error())
 	}
 
+	exists, err := networkExists(networkName)
+	if err != nil {
+		common.LogFail(err.Error())
+	}
+
+	if !exists {
+		common.LogFail(fmt.Sprintf("Network %v does not exist", networkName))
+	}
+
 	for _, containerID := range containerIDs {
 		processType, err := common.DockerInspect(containerID, "{{ index .Config.Labels \"com.dokku.process-type\"}}")
 		if err != nil {