lnd+walletinfo: return more verbose error on DB timeout

Fixes #18. Any bbolt database has a unique lock, meaning it cannot be
opened by two processes at the same time. The simple "timeout" error
that is returned if opening fails is not very informative though.

cmd/chantools/walletinfo.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"go.etcd.io/bbolt"
 	"os"
 	"strings"
 
@@ -140,6 +141,11 @@ func (c *walletInfoCommand) Execute(_ *cobra.Command, _ []string) error {
 		"bdb", lncfg.CleanAndExpandPath(c.WalletDB), false,
 		lnd.DefaultOpenTimeout,
 	)
+	if err == bbolt.ErrTimeout {
+		return fmt.Errorf("error opening wallet database, make sure " +
+			"lnd is not running and holding the exclusive lock " +
+			"on the wallet")
+	}
 	if err != nil {
 		return fmt.Errorf("error opening wallet database: %v", err)
 	}

lnd/channeldb.go

@@ -1,6 +1,7 @@
 package lnd
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"time"
@@ -16,6 +17,11 @@ const (
 
 func OpenDB(dbPath string, readonly bool) (*channeldb.DB, error) {
 	backend, err := openDB(dbPath, false, readonly, DefaultOpenTimeout)
+	if err == bbolt.ErrTimeout {
+		return nil, fmt.Errorf("error opening %s: make sure lnd is "+
+			"not running, database is locked by another process",
+			dbPath)
+	}
 	if err != nil {
 		return nil, err
 	}