Fixes to streaming rollback

* Check fragment removal error code in prepare phase. It is possible
  that the transaction gets BF aborted during fragment removal.
* Mark fragment certified in certify_fragment() even if the provider
  returns cert failed error. With current wsrep-API error codes
  it may not be possible to distinquish certification failure
  and BF abort during fragment replication. This may also be a
  provider bug. As a result rollback fragment may sometimes be
  replicated when it would not be necessary.

include/wsrep/server_service.hpp

@@ -174,6 +174,7 @@ namespace wsrep
          * Provide a server level debug sync point for a caller.
          */
         virtual void debug_sync(const char* sync_point) = 0;
+
     };
 }