[Feature] try to create thumbnail directory if it does not exists

[freechelmi]

When trying to synchronize an old piwigo gallery ( with old thumbnail dir) using the piwigo-videojs plugin , you get an error :

Directory ./galleries/Manel/videos/pwg_representative/ doesn't exist or wrong permission

Maybe the plugin could try to create the directory.

[freechelmi]

the directory seems autocreated actually so closing.

[xbgmsharp]

The directory are not autocreated.
'pwg_representative' is for piwigo > 2.4, 'thumbnail' is for for piwigo < 2.4, as details in wiki https://github.com/xbgmsharp/piwigo-videojs/wiki.
VideoJS poster and Thumbnail generation is not compatible with piwigo <= 2.4 as a release.
The master branch does piwigo <= 2.4, but there might have some bug.
Feel free to test and report.
I might autocreate the 'pwg_representative' or 'thumbnail' direcotry

[xbgmsharp]

Done via commit 305a3ab