MXS-2015: Improve CDC connector error messages

The error messages now display the data received so far. This will help
resolve at which point the streaming process was when an error occurs.

connectors/cdc-connector/cdc_connector.cpp

@@ -28,6 +28,7 @@
 #include <poll.h>
 #include <sstream>
 #include <stdexcept>
+#include <iterator>
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/types.h>
@@ -280,6 +281,11 @@ bool Connection::connect(const std::string& table, const std::string& gtid)
                 {
                     rval = true;
                 }
+                else
+                {
+                    m_error += ". Data received so far: ";
+                    std::copy(m_buffer.begin(), m_buffer.end(), std::back_inserter(m_error));
+                }
             }
         }
     }