Reconvert previews (Re-run conversions) breaks if preview uri not found

Hi,
if u try to reconvert previews for files like image_thumbnail...etc
and the bitcache file for this preview does not exists on server then convertion breaks with message

"The bitcache://xyxyyxyxyxyxxy.... blob was not deleted from the bitcache."

where bitcache://xyxyyxyxyxyxxy is the source file to convert from.

todo:
1. notice about missing bitcache file on server, but dont break the convert process if source file is available
2. change error message bitcache:// uri to real missing bitcache file, not source bitcache file