Handle Persister returning TemporaryFailure for new channels

Previously, if a Persister returned a TemporaryFailure error when
we tried to persist a new channel, the ChainMonitor wouldn't track
the new CHannelMonitor at all, generating a PermanentFailure later
when the updating is restored.

This fixes that by correctly storing the ChannelMonitor on
TemporaryFailures, allowing later update restoration to happen
normally.

This is (indirectly) tested in the next commit where we use
Persister to return all monitor-update errors.

lightning/src/chain/chainmonitor.rs

@@ -367,9 +367,12 @@ where C::Target: chain::Filter,
 				return Err(ChannelMonitorUpdateErr::PermanentFailure)},
 			hash_map::Entry::Vacant(e) => e,
 		};
-		if let Err(e) = self.persister.persist_new_channel(funding_outpoint, &monitor) {
-			log_error!(self.logger, "Failed to persist new channel data");
-			return Err(e);
+		let update_res = self.persister.persist_new_channel(funding_outpoint, &monitor);
+		if update_res.is_err() {
+			log_error!(self.logger, "Failed to persist new channel data: {:?}", update_res);
+		}
+		if update_res == Err(ChannelMonitorUpdateErr::PermanentFailure) {
+			return update_res;
 		}
 		{
 			let funding_txo = monitor.get_funding_txo();
@@ -380,7 +383,7 @@ where C::Target: chain::Filter,
 			}
 		}
 		entry.insert(MonitorHolder { monitor });
-		Ok(())
+		update_res
 	}
 
 	/// Note that we persist the given `ChannelMonitor` update while holding the