Do not fail with a ComponentLookupError when the portlet manager is not found.
Do not try refreshing the portlet when we know this will fail.

This at least happens when our poll portlet is shown in a portlet
manager of collective.panels.
Call @@poll_portlet_render in the current context.

This may be the front-page instead of the Plone site root.
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Merge pull request #69 from collective/maurits-fix-portlet-refresh

Fix portlet refresh