[MM-40712] config: Bugfix on path resolution; fail if given config does not exist (#19360)

* config: bugfix on path resolution; fail if given config does not exist

* reflect review comments

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server_test.go

@@ -168,7 +168,7 @@ func TestStartServerNoS3Bucket(t *testing.T) {
 	s3Endpoint := fmt.Sprintf("%s:%s", s3Host, s3Port)
 
 	s, err := NewServer(func(server *Server) error {
-		configStore, _ := config.NewFileStore("config.json")
+		configStore, _ := config.NewFileStore("config.json", true)
 		store, _ := config.NewStoreFromBacking(configStore, nil, false)
 		server.configStore = store
 		server.UpdateConfig(func(cfg *model.Config) {