Remove future when the query raises an error

[leoasis]

When the query raises an error, remove the future from the list, so that it does not get called again when triggering another future

[leoasis]

The problem here is that we don't know which query is the problematic, since we execute all at once.

[leoasis]

Still, having the problematic queries execute again for next futures won't help, so perhaps the best solution would be to clear the futures queue.

Anyway, common practice to handle errors from db is to clean up, and either start over or just present an error to the user. In any case, no further action should be made with the current transaction, if any, and it must be rollbacked.