Wrap errors with %w when returning them

Many places where we return errors don't use the %w wrapping, because
they predate that functionality.

This patch changes those to use %w, so callers can inspect the
underlying errors with errors.Is and errors.As if needed in the future.

The text of the errors does not change, and no existing callers inspect
these errors, so there are no behaviour changes.

internal/courier/mda.go

@@ -84,7 +84,7 @@ func (p *MDA) Deliver(from string, to string, data []byte) (error, bool) {
 				permanent = status.ExitStatus() != 75
 			}
 		}
-		err = tr.Errorf("MDA delivery failed: %v - %q", err, string(output))
+		err = tr.Errorf("MDA delivery failed: %w - %q", err, string(output))
 		return err, permanent
 	}
 

internal/courier/smtp.go

@@ -160,18 +160,18 @@ func (a *attempt) deliver(mx string) (error, bool) {
 retry:
 	conn, err := net.DialTimeout("tcp", mx+":"+*smtpPort, smtpDialTimeout)
 	if err != nil {
-		return a.tr.Errorf("Could not dial: %v", err), false
+		return a.tr.Errorf("Could not dial: %w", err), false
 	}
 	defer conn.Close()
 	conn.SetDeadline(time.Now().Add(smtpTotalTimeout))
 
 	c, err := smtp.NewClient(conn, mx)
 	if err != nil {
-		return a.tr.Errorf("Error creating client: %v", err), false
+		return a.tr.Errorf("Error creating client: %w", err), false
 	}
 
 	if err = c.Hello(a.courier.HelloDomain); err != nil {
-		return a.tr.Errorf("Error saying hello: %v", err), false
+		return a.tr.Errorf("Error saying hello: %w", err), false
 	}
 
 	secLevel := domaininfo.SecLevel_PLAIN
@@ -235,21 +235,21 @@ retry:
 	}
 
 	if err = c.MailAndRcpt(a.from, a.to); err != nil {
-		return a.tr.Errorf("MAIL+RCPT %v", err), smtp.IsPermanent(err)
+		return a.tr.Errorf("MAIL+RCPT %w", err), smtp.IsPermanent(err)
 	}
 
 	w, err := c.Data()
 	if err != nil {
-		return a.tr.Errorf("DATA %v", err), smtp.IsPermanent(err)
+		return a.tr.Errorf("DATA %w", err), smtp.IsPermanent(err)
 	}
 	_, err = w.Write(a.data)
 	if err != nil {
-		return a.tr.Errorf("DATA writing: %v", err), smtp.IsPermanent(err)
+		return a.tr.Errorf("DATA writing: %w", err), smtp.IsPermanent(err)
 	}
 
 	err = w.Close()
 	if err != nil {
-		return a.tr.Errorf("DATA closing %v", err), smtp.IsPermanent(err)
+		return a.tr.Errorf("DATA closing %w", err), smtp.IsPermanent(err)
 	}
 
 	_ = c.Quit()