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Change PluginDetails to output warning event

Changes PluginDetails to output a warning log event rather than
verbose when a task with no_entries_ok == false produces no entries.
Brings it more in line with similar errors (no input plugins, etc) and
makes problem more obvious in logs.
Merge pull request #121 from theaquamarine/plugin_warning

Change PluginDetails to output warning event
Merge pull request #119 from tarzasai/subtitles

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