Save error on first failure.
    
At end of execute, if the current error is 4350 (transaction already aborted), restore
the original error (if there was one).


ndb/src/ndbapi/NdbConnection.cpp:
  In NdbConnection::execute - returning 4350 instead of correct error.
  Save error on first failure.
      
  At end of execute, if the current error is 4350 (transaction already aborted), restore
  the original error (if there was one).