groonga alone: ensure clearing error for each command execution
groonga alone: ensure clearing error for each command execution

Generally, we don't need this because GRN_API_ENTER clear error and
GRN_CANCEL isn't occurred in alone mode (single thread).

groonga/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga