lnpeer+peer: use importable lnpeer.ErrPeerExiting

As a prepatory step to making gossip replies synchronous, we will move
the ErrPeerExiting error into the lnpeer package so that it can be
imported by the discovery package. With synchronous sends, this error
can now be detected and handled by goroutines in the syncer, and cause
them to exit instead of continue to sending backlogs.

lnpeer/errors.go

@@ -0,0 +1,8 @@
+package lnpeer
+
+import "fmt"
+
+var (
+	// ErrPeerExiting signals that the peer received a disconnect request.
+	ErrPeerExiting = fmt.Errorf("peer exiting")
+)