Missing message is now 404 instead of 500

- Added smtpd.ErrNotExist to make detecting missing message easier
- Return 404 instead of 500 when requesting a non-extistant message
- More REST unit tests

smtpd/datastore.go

@@ -1,12 +1,15 @@
 package smtpd
 
 import (
+	"errors"
 	"github.com/jhillyerd/go.enmime"
 	"io"
 	"net/mail"
 	"time"
 )
 
+var ErrNotExist = errors.New("Message does not exist")
+
 type DataStore interface {
 	MailboxFor(emailAddress string) (Mailbox, error)
 	AllMailboxes() ([]Mailbox, error)