channeldb: add doc strings to finalize funcs/structs

channeldb/error.go

@@ -3,6 +3,8 @@ package channeldb
 import "fmt"
 
 var (
-	ErrNoExists         = fmt.Errorf("channel db has not yet been created")
+	ErrNoChanDBExists = fmt.Errorf("channel db has not yet been created")
+
 	ErrNoActiveChannels = fmt.Errorf("no active channels exist")
+	ErrChannelNoExist   = fmt.Errorf("this channel does not exist")
 )