Add support to migrate from gogs (#14342)

Add support to migrate gogs:

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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

modules/migrations/base/error.go

@@ -0,0 +1,26 @@
+// Copyright 2021 The Gitea 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 base
+
+import "fmt"
+
+// ErrNotSupported represents status if a downloader do not supported something.
+type ErrNotSupported struct {
+	Entity string
+}
+
+// IsErrNotSupported checks if an error is an ErrNotSupported
+func IsErrNotSupported(err error) bool {
+	_, ok := err.(ErrNotSupported)
+	return ok
+}
+
+// Error return error message
+func (err ErrNotSupported) Error() string {
+	if len(err.Entity) != 0 {
+		return fmt.Sprintf("'%s' not supported", err.Entity)
+	}
+	return "not supported"
+}