Add general invalid message

All minipots should implement these types of messages:

• connect
• login
• invalid

(some would have some extra message types).

These message types corresponds:

• connect: An opened connection. No transferred data is needed to generate this message
• login: "Valid" login was attempted. username and password is needed to send in the data
• invalid: Any error occured, e.g. invalid encoding, broken format, unexpected data, etc. This should cover overall error

Later, we would add e.g. invalid_login to distinguish general error and error that occurred during login process.