Fix #65: report errors in tarballs/zipfiles to the user (#68)
This is a bit of a hack in comparison with the normal validation
method, but these errors happen on upload, where the other errors
happen on validation.
Sadly, the tusd implementation is a bit wonky, and we cannot really
communicate back with the client when we found such error during
upload (as strictly seen the upload succeeded fine; the content
is broken). So we cannot report the error on upload, and as such
are forced to do it after upload.