Use fmt.Errorf() (gosimple)

This fixes:
pkg/cmd/grafana-cli/commands/install_command.go:36:11: should use fmt.Errorf(...) instead of errors.New(fmt.Sprintf(...)) (S1028)
pkg/models/org_user.go:53:11: should use fmt.Errorf(...) instead of errors.New(fmt.Sprintf(...)) (S1028)
pkg/services/notifications/mailer.go:138:16: should use fmt.Errorf(...) instead of errors.New(fmt.Sprintf(...)) (S1028)

pkg/cmd/grafana-cli/commands/install_command.go

@@ -33,7 +33,7 @@ func validateInput(c CommandLine, pluginFolder string) error {
 	fileInfo, err := os.Stat(pluginsDir)
 	if err != nil {
 		if err = os.MkdirAll(pluginsDir, os.ModePerm); err != nil {
-			return errors.New(fmt.Sprintf("pluginsDir (%s) is not a writable directory", pluginsDir))
+			return fmt.Errorf("pluginsDir (%s) is not a writable directory", pluginsDir)
 		}
 		return nil
 	}

pkg/models/org_user.go

@@ -50,7 +50,7 @@ func (r *RoleType) UnmarshalJSON(data []byte) error {
 
 	if !(*r).IsValid() {
 		if (*r) != "" {
-			return errors.New(fmt.Sprintf("JSON validation error: invalid role value: %s", *r))
+			return fmt.Errorf("JSON validation error: invalid role value: %s", *r)
 		}
 
 		*r = ROLE_VIEWER

pkg/services/notifications/mailer.go

@@ -7,7 +7,6 @@ package notifications
 import (
 	"bytes"
 	"crypto/tls"
-	"errors"
 	"fmt"
 	"html/template"
 	"net"
@@ -135,7 +134,7 @@ func buildEmailMessage(cmd *m.SendEmailCommand) (*Message, error) {
 		subjectText, hasSubject := subjectData["value"]
 
 		if !hasSubject {
-			return nil, errors.New(fmt.Sprintf("Missing subject in Template %s", cmd.Template))
+			return nil, fmt.Errorf("Missing subject in Template %s", cmd.Template)
 		}
 
 		subjectTmpl, err := template.New("subject").Parse(subjectText.(string))