make polkadot die graciously (#2056)

While investigating some db migrations that make the node startup fail,
I noticed that the node wasn't exiting and that the log file were
growing exponentially, until my whole system was freezing and that makes
it really hard to actually find why it was failing in the first place.

E.g:
```
 ls -lh /tmp/zombie-01a04c2a2c0265d85f6440cf01c0f44a_-51319-uyggzuD4wEpV/bob.log
 32,6G oct 27 11:16 /tmp/zombie-01a04c2a2c0265d85f6440cf01c0f44a_-51319-uyggzuD4wEpV/bob.log
```

This was happening because the following errors were being printed
continously without the subsystem main loop exiting:

From dispute-coordinator:
```
WARN tokio-runtime-worker parachain::dispute-coordinator: error=Subsystem(Generated(Context("Signal channel is terminated and empty.")))
```

From availability recovery:
```
Erasure task channel closed. Node shutting down ?
```

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

polkadot/node/core/dispute-coordinator/src/lib.rs

@@ -27,6 +27,7 @@
 
 use std::sync::Arc;
 
+use error::FatalError;
 use futures::FutureExt;
 
 use gum::CandidateHash;
@@ -431,7 +432,7 @@ impl DisputeCoordinatorSubsystem {
 #[overseer::contextbounds(DisputeCoordinator, prefix = self::overseer)]
 async fn wait_for_first_leaf<Context>(ctx: &mut Context) -> Result<Option<ActivatedLeaf>> {
 	loop {
-		match ctx.recv().await? {
+		match ctx.recv().await.map_err(FatalError::SubsystemReceive)? {
 			FromOrchestra::Signal(OverseerSignal::Conclude) => return Ok(None),
 			FromOrchestra::Signal(OverseerSignal::ActiveLeaves(update)) => {
 				if let Some(activated) = update.activated {

polkadot/node/network/availability-recovery/src/lib.rs

@@ -822,6 +822,7 @@ async fn erasure_task_thread(
 					target: LOG_TARGET,
 					"Erasure task channel closed. Node shutting down ?",
 				);
+				break
 			},
 		}
 	}