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/filestore.go

@@ -176,7 +176,7 @@ func (mb *FileMailbox) GetMessage(id string) (Message, error) {
 		}
 	}
 
-	return nil, fmt.Errorf("Message %s not in index", id)
+	return nil, ErrNotExist
 }
 
 // Delete all messages in this mailbox