multi: use the "errors" package everywhere

Replace all usages of the "github.com/go-errors/errors" and
"github.com/pkg/errors" packages with the standard lib's "errors"
package. This ensures that error wrapping and `errors.Is` checks will
work as expected.

htlcswitch/switch_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/sha256"
+	"errors"
 	"fmt"
 	"io"
 	mrand "math/rand"
@@ -13,7 +14,6 @@ import (
 
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/davecgh/go-spew/spew"
-	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/contractcourt"