fix(project background): add more checks for whether a background file exists when duplicating or deleting a project

Related discussion: https://community.vikunja.io/t/500-internal-server-error-when-selecting-unsplash-background-image/778/18

pkg/models/project.go

@@ -1030,7 +1030,12 @@ func (p *Project) DeleteBackgroundFileIfExists() (err error) {
 	}
 
 	file := files.File{ID: p.BackgroundFileID}
-	return file.Delete()
+	err = file.Delete()
+	if err != nil && files.IsErrFileDoesNotExist(err) {
+		return nil
+	}
+
+	return err
 }
 
 // SetProjectBackground sets a background file as project background in the db