This is the function that is specific to your backend. It takes

the current password (as supplied by the user) and the desired new password. It will return an array of messages. If everything was successful, the array will be empty. Else, it will contain the errormessage(s). Constants to be used for these messages: CPW_CURRENT_NOMATCH -> "Your current password is not correct." CPW_INVALID_PW -> "Your new password contains invalid characters."

• return: Array of error messages.