Refactor error handling for nil template pointers

Replace fmt.Errorf with errors.New for consistency in error handling. This change ensures a more streamlined and uniform error reporting approach across the codebase. Also updated a test case from t.Errorf to t.Error for better readability.

b64linebreaker.go

@@ -5,7 +5,7 @@
 package mail
 
 import (
-	"fmt"
+	"errors"
 	"io"
 )
 
@@ -26,7 +26,7 @@ var newlineBytes = []byte(SingleNewLine)
 // line length is reached
 func (l *Base64LineBreaker) Write(data []byte) (numBytes int, err error) {
 	if l.out == nil {
-		err = fmt.Errorf(ErrNoOutWriter)
+		err = errors.New(ErrNoOutWriter)
 		return
 	}
 	if l.used+len(data) < MaxBodyLength {

msg.go

@@ -743,7 +743,7 @@ func (m *Msg) SetBodyWriter(
 // The content type will be set to text/html automatically
 func (m *Msg) SetBodyHTMLTemplate(tpl *ht.Template, data interface{}, opts ...PartOption) error {
 	if tpl == nil {
-		return fmt.Errorf(errTplPointerNil)
+		return errors.New(errTplPointerNil)
 	}
 	buffer := bytes.Buffer{}
 	if err := tpl.Execute(&buffer, data); err != nil {
@@ -758,7 +758,7 @@ func (m *Msg) SetBodyHTMLTemplate(tpl *ht.Template, data interface{}, opts ...Pa
 // The content type will be set to text/plain automatically
 func (m *Msg) SetBodyTextTemplate(tpl *tt.Template, data interface{}, opts ...PartOption) error {
 	if tpl == nil {
-		return fmt.Errorf(errTplPointerNil)
+		return errors.New(errTplPointerNil)
 	}
 	buf := bytes.Buffer{}
 	if err := tpl.Execute(&buf, data); err != nil {
@@ -790,7 +790,7 @@ func (m *Msg) AddAlternativeWriter(
 // The content type will be set to text/html automatically
 func (m *Msg) AddAlternativeHTMLTemplate(tpl *ht.Template, data interface{}, opts ...PartOption) error {
 	if tpl == nil {
-		return fmt.Errorf(errTplPointerNil)
+		return errors.New(errTplPointerNil)
 	}
 	buffer := bytes.Buffer{}
 	if err := tpl.Execute(&buffer, data); err != nil {
@@ -805,7 +805,7 @@ func (m *Msg) AddAlternativeHTMLTemplate(tpl *ht.Template, data interface{}, opt
 // The content type will be set to text/plain automatically
 func (m *Msg) AddAlternativeTextTemplate(tpl *tt.Template, data interface{}, opts ...PartOption) error {
 	if tpl == nil {
-		return fmt.Errorf(errTplPointerNil)
+		return errors.New(errTplPointerNil)
 	}
 	buffer := bytes.Buffer{}
 	if err := tpl.Execute(&buffer, data); err != nil {
@@ -1314,7 +1314,7 @@ func fileFromReadSeeker(name string, reader io.ReadSeeker) *File {
 // fileFromHTMLTemplate returns a File pointer form a given html/template.Template
 func fileFromHTMLTemplate(name string, tpl *ht.Template, data interface{}) (*File, error) {
 	if tpl == nil {
-		return nil, fmt.Errorf(errTplPointerNil)
+		return nil, errors.New(errTplPointerNil)
 	}
 	buffer := bytes.Buffer{}
 	if err := tpl.Execute(&buffer, data); err != nil {
@@ -1326,7 +1326,7 @@ func fileFromHTMLTemplate(name string, tpl *ht.Template, data interface{}) (*Fil
 // fileFromTextTemplate returns a File pointer form a given text/template.Template
 func fileFromTextTemplate(name string, tpl *tt.Template, data interface{}) (*File, error) {
 	if tpl == nil {
-		return nil, fmt.Errorf(errTplPointerNil)
+		return nil, errors.New(errTplPointerNil)
 	}
 	buffer := bytes.Buffer{}
 	if err := tpl.Execute(&buffer, data); err != nil {

msgwriter_test.go

@@ -122,7 +122,7 @@ func TestMsgWriter_writeMsg(t *testing.T) {
 			em += fmt.Sprintf("* incorrect %q field", ea[e])
 		}
 		em += fmt.Sprintf("\n\nFull message:\n%s", ms)
-		t.Errorf(em)
+		t.Error(em)
 	}
 }