Additional API support for labels (#3290)

* Add API support for labels.

* Error handling for adding/replacing multiple issue labels

* Revisions to function names and error handling. Use issue.ClearLabels in replace/clear functions

* Additional code cleanup

models/error.go

@@ -5,6 +5,7 @@
 package models
 
 import (
+	"bytes"
 	"fmt"
 )
 
@@ -34,6 +35,30 @@ func (err ErrNamePatternNotAllowed) Error() string {
 	return fmt.Sprintf("name pattern is not allowed [pattern: %s]", err.Pattern)
 }
 
+type ErrMultipleErrors struct {
+	Errors []error
+}
+
+func IsErrMultipleErrors(err error) bool {
+	_, ok := err.(ErrMultipleErrors)
+	return ok
+}
+
+func (err ErrMultipleErrors) Error() string {
+	var message bytes.Buffer
+
+	message.WriteString("Multiple errors encountered: ")
+
+	for i := range err.Errors {
+		message.WriteString(err.Errors[i].Error())
+		if i < len(err.Errors)-1 {
+			message.WriteString("; ")
+		}
+	}
+
+	return message.String()
+}
+
 //  ____ ___
 // |    |   \______ ___________
 // |    |   /  ___// __ \_  __ \
@@ -545,6 +570,20 @@ func (err ErrLabelNotExist) Error() string {
 	return fmt.Sprintf("label does not exist [id: %d]", err.ID)
 }
 
+type ErrLabelNotValidForRepository struct {
+	ID     int64
+	RepoID int64
+}
+
+func IsErrLabelNotValidForRepository(err error) bool {
+	_, ok := err.(ErrLabelNotValidForRepository)
+	return ok
+}
+
+func (err ErrLabelNotValidForRepository) Error() string {
+	return fmt.Sprintf("label is not valid for repository [label_id: %d, repo_id: %d]", err.ID, err.RepoID)
+}
+
 //    _____  .__.__                   __
 //   /     \ |__|  |   ____   _______/  |_  ____   ____   ____
 //  /  \ /  \|  |  | _/ __ \ /  ___/\   __\/  _ \ /    \_/ __ \