Fix crash in share Media, fix failed download media

[MV-GH]

Learned quite a bit from this. The original share/download since the introduction of the caching check was only really working in some instances due to some quirks.

mistakes:

• scope.launch won't rethrow unhandled exceptions but will cancel its parents thus bypassing the try catch I had
• shareMedia used scope.launch for the IO part but it did not actually wait for this complete, thus it was sharing empty files at best crash at worst due to two sources reading/write to a cache file (only really worked in the rare instance its called from imageviewer where the image is loaded in cache and it wins the race condition on reading the file into the cached file)
• Context.getInputStream shared a closed inputStream, the implementation for snapshot doesn't actually check if it is closed thus that worked. But the byteStream implementation did. Thus the cause of the IOException: Closed exception

Fixes #1204

[dessalines]

LGTM