A Cookbook download whose repo/quant selector matched no files (e.g. a
':Q4_K_M' tag that does not exist) printed 'Fetching 0 files' and was still
reported as a successful '✓ Downloaded' / completed task. Detect the
zero-file signature in the download snapshot and mark the task as an error
with a clear diagnosis (no matching files — check the repo or quant/filename
pattern) so users know nothing was actually downloaded. Normal multi-file
and fully-cached downloads (which print 'Fetching N files', N>0) are
unaffected.
Closes#839
63a947d246