fix(jellyfinlogin): add proper error message when no admin user exists (#1216)

This PR adds an error message when the database has no admin user and Jellyseerr has already been
set up (i.e. settings.json is filled in), instead of having a generic error message.

server/constants/error.ts

@@ -4,6 +4,7 @@ export enum ApiErrorCode {
   InvalidAuthToken = 'INVALID_AUTH_TOKEN',
   InvalidEmail = 'INVALID_EMAIL',
   NotAdmin = 'NOT_ADMIN',
+  NoAdminUser = 'NO_ADMIN_USER',
   SyncErrorGroupedFolders = 'SYNC_ERROR_GROUPED_FOLDERS',
   SyncErrorNoLibraries = 'SYNC_ERROR_NO_LIBRARIES',
   Unknown = 'UNKNOWN',