Implement Go 1.13 error interfaces (flyteorg#44)

* Implement Go 1.13 error interfaces

* Update comment

flytestdlib/errors/error.go

@@ -23,6 +23,16 @@ func (e *err) Code() ErrorCode {
 	return e.code
 }
 
+// Overrides Is to check for error code only. This enables the default package's errors.Is().
+func (e *err) Is(target error) bool {
+	t, ok := target.(*err)
+	if !ok {
+		return false
+	}
+
+	return e.Code() == t.Code()
+}
+
 type errorWithCause struct {
 	*err
 	cause error
@@ -36,6 +46,11 @@ func (e *errorWithCause) Cause() error {
 	return e.cause
 }
 
+// Overrides Unwrap to retrieve the underlying error. This enables the default package's errors.Unwrap().
+func (e *errorWithCause) Unwrap() error {
+	return e.Cause()
+}
+
 // Creates a new error using an error code and a message.
 func Errorf(errorCode ErrorCode, msgFmt string, args ...interface{}) error {
 	return &err{

flytestdlib/errors/error_test.go

@@ -1,6 +1,7 @@
 package errors
 
 import (
+	"errors"
 	"fmt"
 	"testing"
 
@@ -45,3 +46,23 @@ func TestIsCausedByError(t *testing.T) {
 	assert.True(t, IsCausedByError(e2, eRoot))
 	assert.True(t, IsCausedByError(e2, e1))
 }
+
+func TestErrorsIs(t *testing.T) {
+	eRoot := Errorf("Code1", "msg")
+	assert.True(t, errors.Is(eRoot, Errorf("Code1", "different msg")))
+
+	e1 := Wrapf("Code2", eRoot, "Wrapped error")
+	assert.True(t, errors.Is(e1, Errorf("Code1", "different msg")))
+}
+
+func TestErrorsUnwrap(t *testing.T) {
+	eRoot := Errorf("Code1", "msg")
+	e1 := Wrapf("Code2", eRoot, "Wrapped error")
+	assert.True(t, errors.Is(e1, Errorf("Code1", "different msg")))
+
+	newErr := &err{}
+	assert.True(t, errors.As(e1, &newErr))
+	assert.Equal(t, "Code1", newErr.Code())
+
+	assert.True(t, errors.Is(errors.Unwrap(e1), Errorf("Code1", "different msg")))
+}