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.

shachain/element_test.go

@@ -1,10 +1,10 @@
 package shachain
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 
-	"github.com/go-errors/errors"
 	"github.com/stretchr/testify/require"
 )