WIP Unify error codes returned to the connection handler

Error argument in the connection handler is not always set and in some
cases provides meaningless values. For example, every TLS module returns
its specific error codes.

Current solution unifies error codes and allows to distinguish between
different types of disconnection.

Fixes #123.

strophe.h

@@ -103,6 +103,26 @@ extern "C" {
  *  Internal failure error code.
  */
 #define XMPP_EINT -3
+/** @def XMPP_ECERT
+ *  Certificate verification failed during TLS establishment.
+ */
+#define XMPP_ECERT -4
+/** @def XMPP_ETIMEDOUT
+ *  Operation timed out error code.
+ */
+#define XMPP_ETIMEDOUT -5
+/** @def XMPP_EIO
+ *  I/O error code.
+ */
+#define XMPP_EIO -6
+/** @def XMPP_ERESET
+ *  Error code for situation when connection closed by remote side.
+ */
+#define XMPP_ERESET -7
+/** @def XMPP_EAUTH
+ *  Authentication failed error code.
+ */
+#define XMPP_EAUTH -8
 
 /* initialization and shutdown */
 void xmpp_initialize(void);