message: clearer error message when using schema in column ref (which is unsupported)
diff: @@ -12,7 +12,7 @@ unique_ptr<ParsedExpression> Transformer::TransformColumnRef(PGColumnRef *root)
  auto fields = root->fields;
  switch ((reinterpret_cast<PGNode *>(fields->head->data.ptr_value))->type) {
  case T_PGString: {
  - if (fields->length < 1 || fields->length > 2) {
  + if (fields->length < 1) {
  throw ParserException("Unexpected field length");
  }
  string column_name, table_name;
  @@ -21,7 +21,7 @@ unique_ptr<ParsedExpression> Transformer::TransformColumnRef(PGColumnRef *root)
  auto colref = make_unique<ColumnRefExpression>(column_name, table_name);
  colref->query_location = root->location;
  return move(colref);
  - } else {
  + } else if (fields->length == 2) {
  table_name = string(reinterpret_cast<PGValue *>(fields->head->data.ptr_value)->val.str);
  auto col_node = reinterpret_cast<PGNode *>(fields->head->next->data.ptr_value);
  switch (col_node->type) {
  @@ -37,6 +37,8 @@ unique_ptr<ParsedExpression> Transformer::TransformColumnRef(PGColumnRef *root)
  default:
  throw NotImplementedException("ColumnRef not implemented!");
  }
  + } else {
  + throw NotImplementedException("ColumnRef not implemented!");
  }
  }
  case T_PGAStar: {
  