Improve loading speed of desktop app (#4865)

# Description of Changes
Improve loading speed of desktop app by loading a default config until
the backend has spawned.

frontend/src/desktop/constants/backendErrors.ts

@@ -0,0 +1,20 @@
+import i18n from '@app/i18n';
+
+export const BACKEND_NOT_READY_CODE = 'BACKEND_NOT_READY' as const;
+
+export interface BackendNotReadyError extends Error {
+  code: typeof BACKEND_NOT_READY_CODE;
+}
+
+export function createBackendNotReadyError(): BackendNotReadyError {
+  return Object.assign(new Error(i18n.t('backendHealth.starting', 'Backend starting up...')), {
+    code: BACKEND_NOT_READY_CODE,
+  });
+}
+
+export function isBackendNotReadyError(error: unknown): error is BackendNotReadyError {
+  return typeof error === 'object'
+    && error !== null
+    && 'code' in error
+    && (error as { code?: unknown }).code === BACKEND_NOT_READY_CODE;
+}