feat: skip scaled processes that are missing in the Procfile

This allows folks to deploy apps that don't have a web process without needing to scale that process down before/after the first deploy. Note that the formations key in the app.json or a manual scale of other processes will be necessary to start anything non-web.

Closes #5700

plugins/ps/triggers.go

@@ -1,6 +1,7 @@
 package ps
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"path"
@@ -259,6 +260,15 @@ func TriggerPreDeploy(appName string, imageTag string) error {
 	return nil
 }
 
+// TriggerProcfileExists checks if a procfile exists
+func TriggerProcfileExists(appName string) error {
+	if hasProcfile(appName) {
+		return nil
+	}
+
+	return errors.New("Procfile does not exist")
+}
+
 // TriggerProcfileGetCommand fetches a command from the procfile
 func TriggerProcfileGetCommand(appName string, processType string, port int) error {
 	if !hasProcfile(appName) {