Create a general solution for xmpp_conn_handler error code responses

[matisszilard]

Description

The xmpp_conn_handler function is notified when a connection related event occurred.

typedef void (*xmpp_conn_handler)(xmpp_conn_t * const conn, 
				  const xmpp_conn_event_t event,
				  const int error,
				  xmpp_stream_error_t * const stream_error,
				  void * const userdata);

There are some cases, when the error code or the stream_error parameters are not set.
Ex.: there is an authentication error (source file), or a socket error (source file).

In some cases the error code is set properly. Ex.: tls error (source code), connection abort error etc.

Possible solution

Create an enum with the possible error codes / events and set as the connection struct error parameter. The user application can decide how to handle the given situation.

Example.:

enum StropheErrorCode {
    NO_ERROR = 0,
    AUTHENTICATION_FAILURE = 1,
    CONNECTION_TIMEOUT = 2,
    TLS_ERROR = 3,
    CONNECTION_FAIL = 4
    ...
};

Connected issue:

#122