Issue/638 small thumbnail sharing media

This patch just add a try/catch, and better explain/organize the code that resizes picture when shared from Media Section.

It's NOT a real fix for this issue, since for self-hosted blog we are relying on the thumbnail URL, instead of FileURL, for performaces/OOM issues. 
Thumbnail URL usually points to a really small version of the picture (it depends on server side blog settings).

If we really want to fix this issue, we need to download the picture in a File in a tmp location, open a stream on the file, and resize the picture on-fly by using the same code of PostUloadingService.