Suppress error while failed to run serf event handler.

The error is:
message/error/missing-dataset.test: 2014-12-16T10:09:33+00:00[6290][error]: serf[6295]: agent: Error invoking script 'droonga-engine-serf-event-handler': exit status 1
SUCCESS

It appears when there is globally installed droonga-engine and we are running tests in a local repository.
(Serf tries to start glbally installed droonga-engine-serf-event-handler command and it tries to touch files under ~droonga-engine even if the current user is not the user, then it fails with "permission denied" error.)