Merge pull request #1474 from hedgedoc/failing-migrations-notice

[1.x] Add help link and short explanation for failing migrations

lib/models/index.js

@@ -82,7 +82,10 @@ db.runMigrations = async function runMigrations () {
   // exit in case of unsuccessful migrations
   await umzug.up().catch(error => {
     logger.error(error)
-    logger.error('Database migration failed. Exiting…')
+    logger.error(`Database migration failed.
+This can be the result of upgrading from quite old versions and requires manual database intervention.
+See https://docs.hedgedoc.org/guides/migration-troubleshooting/ for help.
+Exiting…`)
     process.exit(1)
   })
   logger.info('All migrations performed successfully')