feat: add app name verification to all golang plugins

Moving verification of input to the earliest place is the best way to decrease the need to validate this further downstream.

Additionally, some of the existing checks were.... lacking :)

plugins/repo/subcommands.go

@@ -1,18 +1,12 @@
 package repo
 
 import (
-	"errors"
-
 	"github.com/dokku/dokku/plugins/common"
 )
 
 // CommandGc runs 'git gc --aggressive' against the application's repo
 func CommandGc(appName string) error {
-	if appName == "" {
-		return errors.New("Please specify an app to run the command on")
-	}
-	err := common.VerifyAppName(appName)
-	if err != nil {
+	if err := common.VerifyAppName(appName); err != nil {
 		return err
 	}
 
@@ -28,8 +22,8 @@ func CommandGc(appName string) error {
 
 // CommandPurgeCache deletes the contents of the build cache stored in the repository
 func CommandPurgeCache(appName string) error {
-	if appName == "" {
-		common.LogFail("Please specify an app to run the command on")
+	if err := common.VerifyAppName(appName); err != nil {
+		return err
 	}
 
 	return PurgeCache(appName)