Test Send error message

This test is added to ensure consistent error messages as some users of
the package are currently parsing it in order to extract the cause and
index of errors in batches. Later commits fix this usability issue by exporting
a new SendError type.

Author: ivy

send_test.go

@@ -2,6 +2,7 @@ package mail
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"reflect"
 	"testing"
@@ -50,6 +51,18 @@ func TestSend(t *testing.T) {
 	}
 }
 
+func TestSendError(t *testing.T) {
+	s := &mockSendCloser{
+		mockSender: func(_ string, _ []string, _ io.WriterTo) error {
+			return errors.New("kaboom")
+		},
+	}
+	wantErr := "gomail: could not send email 1: kaboom"
+	if err := Send(s, getTestMessage()); err == nil || err.Error() != wantErr {
+		t.Errorf("expected Send() error, got %q, want %q", err, wantErr)
+	}
+}
+
 func getTestMessage() *Message {
 	m := NewMessage()
 	m.SetHeader("From", testFrom)