MXS-1886 Better auto-rejoin error description and tolerance

Contains changes from commit 09df01752812444c6e7c409a8957d292f7de63cf
adapted to the 2.3 branch.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -313,9 +313,10 @@ public:
      * The non-detected errors will mostly be detected once the slave tries to start replicating.
      *
      * @param master_info Master server
+     * @param error_out Details the reason for a negative result
      * @return True if slave can replicate from master
      */
-    bool can_replicate_from(MariaDBServer* master);
+    bool can_replicate_from(MariaDBServer* master, std::string* error_out);
 
     /**
      * Redirect one slave server to another master