MGL-301 Generalize message for error_connection_failed

In practice WSREP_CONN_FAIL provider error code came to mean inability
to replicate writesets, and this applies not only to a situation where
the node is not connected to a primary component, but also when it is
not sufficiently synced - e.g. is receiving an SST or lags behind too
much. Change the error message from "not connected" to "not synced" to
cover all such situations.

src/provider.cpp

@@ -72,7 +72,7 @@ wsrep::provider::to_string(enum wsrep::provider::status const val)
     case error_size_exceeded:
         return "Transaction size exceeded";
     case error_connection_failed:
-        return "Not connected to Primary Component";
+        return "Not synced with a Primary Component";
     case error_provider_failed:
         return "Provider in bad state, needs to be reinitialized.";
     case error_fatal: