Handle missing version file or package manifest

[Varneon]

If trying to open an export configuration with both version file and package manifest missing, editor will throw a NullReferenceException:

NullReferenceException: Object reference not set to an instance of an object
Varneon.PackageExporter.PackageFileNameUtility.GenerateName (Varneon.PackageExporter.PackageExportConfiguration configuration, System.String versionString) (at Packages/com.varneon.package-exporter/Editor/PackageFileNameUtility.cs:14)
Varneon.PackageExporter.PackageExportConfiguration.GenerateFileName (System.String versionString) (at Packages/com.varneon.package-exporter/Editor/PackageExportConfiguration.cs:153)
Varneon.PackageExporter.ExporterWindow.UpdateOutputFileName () (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:756)
Varneon.PackageExporter.ExporterWindow.SetActiveConfiguration (System.String name) (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:539)
Varneon.PackageExporter.ExporterWindow.LoadPackageConfigurations () (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:501)
Varneon.PackageExporter.ExporterWindow.LoadPackageConfigurationStorages (System.Boolean createNewIfNoneFound) (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:473)
Varneon.PackageExporter.ExporterWindow.OnEnable () (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:308)
UnityEditor.EditorWindow:GetWindow()
Varneon.PackageExporter.ExporterWindow:OpenWindow() (at Packages/com.varneon.package-exporter/Editor/ExporterWindow.cs:296)

Add error handling for this and display a notification for the user link either version file or package manifest to the configuration