Fix confusing error in database restore

If a database restore failed, users would see a confusing error message:

```
ArgumentError (wrong number of arguments (given 2, expected 0..1))
```

The `abort` call only takes a string, so simplify the call accordingly.

lib/backup/database.rb

@@ -71,7 +71,7 @@ module Backup
       end
 
       report_success(success)
-      abort Backup::Error, 'Restore failed' unless success
+      abort 'Restore failed' unless success
     end
 
     protected