channeldb: add limit on the max number of opaque bytes per announcement

In this commit, we add a new limit on the largest number of extra opaque
bytes that we'll allow to be written per vertex/edge. We do this in
order to limit the amount of disk space that we expose, as it's possible
that nodes may start to pad their announcements adding an additional
externalized cost as nodes may need to continue to store and relay these
large announcements.

channeldb/error.go

@@ -94,3 +94,13 @@ var (
 	// to the log not having any recorded events.
 	ErrNoForwardingEvents = fmt.Errorf("no recorded forwarding events")
 )
+
+// ErrTooManyExtraOpaqueBytes creates an error which should be returned if the
+// caller attempts to write an announcement message which bares too many extra
+// opaque bytes. We limit this value in order to ensure that we don't waste
+// disk space due to nodes unnecessarily padding out their announcements with
+// garbage data.
+func ErrTooManyExtraOpaqueBytes(numBytes int) error {
+	return fmt.Errorf("max allowed number of opaque bytes is %v, received "+
+		"%v bytes", MaxAllowedExtraOpaqueBytes, numBytes)
+}