Issue #2228885 by Sweetchuck: Error message is attached to a non exists...

Issue #2228885 by Sweetchuck: Error message is attached to a non exists element  when table prefix is wrong.

core/lib/Drupal/Core/Database/Install/Tasks.php

@@ -299,7 +299,7 @@ public function validateDatabaseSettings($database) {
 
     // Verify the table prefix.
     if (!empty($database['prefix']) && is_string($database['prefix']) && !preg_match('/^[A-Za-z0-9_.]+$/', $database['prefix'])) {
-      $errors[$database['driver'] . '][advanced_options][db_prefix'] = t('The database table prefix you have entered, %prefix, is invalid. The table prefix can only contain alphanumeric characters, periods, or underscores.', array('%prefix' => $database['prefix']));
+      $errors[$database['driver'] . '][prefix'] = t('The database table prefix you have entered, %prefix, is invalid. The table prefix can only contain alphanumeric characters, periods, or underscores.', array('%prefix' => $database['prefix']));
     }
 
     return $errors;