Update import paths from github.com/go-gitea to code.gitea.io (#135)

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

vendor/code.gitea.io/git/error.go

@@ -0,0 +1,50 @@
+// Copyright 2015 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package git
+
+import (
+	"fmt"
+	"time"
+)
+
+type ErrExecTimeout struct {
+	Duration time.Duration
+}
+
+func IsErrExecTimeout(err error) bool {
+	_, ok := err.(ErrExecTimeout)
+	return ok
+}
+
+func (err ErrExecTimeout) Error() string {
+	return fmt.Sprintf("execution is timeout [duration: %v]", err.Duration)
+}
+
+type ErrNotExist struct {
+	ID      string
+	RelPath string
+}
+
+func IsErrNotExist(err error) bool {
+	_, ok := err.(ErrNotExist)
+	return ok
+}
+
+func (err ErrNotExist) Error() string {
+	return fmt.Sprintf("object does not exist [id: %s, rel_path: %s]", err.ID, err.RelPath)
+}
+
+type ErrUnsupportedVersion struct {
+	Required string
+}
+
+func IsErrUnsupportedVersion(err error) bool {
+	_, ok := err.(ErrUnsupportedVersion)
+	return ok
+}
+
+func (err ErrUnsupportedVersion) Error() string {
+	return fmt.Sprintf("Operation requires higher version [required: %s]", err.Required)
+}