Halt backfill improvements

[fabriziodemaria]

Halting a backfill causes all backfill's instances to be halted. Failing to halt even a single instance will stop the halting procedure giving to the user a message for half-success (backfill is halted, but some instances are still running). We should retry to halt an instance in case of error.

[ulzha]

Yeah, backfill shouldn't change status to halted though unless all the instances were halted. The choice to retry or not perhaps should be left to the client, otherwise we can end up retrying for arbitrarily long time...

[honnix]

Failing to halt even a single instance will stop the halting procedure

This is not anymore the case. Styx will try to halt all instances and return an error to the client if failed to halt any of the instances.