UMP sink bug: XCM message would be discarded if ump_service_total_weight reached

[shaunxw]

The message should be put back to queue if execution failed because of weight exhausted (which is caused by ump_service_total_weight limitation in host config). Currently it's discarded, which would lead to fund loss.

polkadot/runtime/parachains/src/ump.rs

Lines 419 to 424 in 285a35b

	Err((id, required)) => {
	// we process messages in order and don't drop them if we run out of weight, so need to break
	// here.
	Self::deposit_event(Event::WeightExhausted(id, max_weight, required));
	break
	},

Fortunately this hasn't happened yet, as ump_service_total_weight is set to 100_000_000_000, which makes it be able to handle 33 orml-xtokens transfer in one block. max_upward_message_num_per_candidate is set to 10, and since we have only Karura and Statemine sending ump transfers/teleports for now, this limitation hasn't be reached yet.

But it needs to be fixed ASAP.

[shawntabrizi]

@pepyakin °

[gavofyork]

I'll take it.