removing files superceded by utils0-.tst - #108

A download interrupted halfway currently starts again from zero,
which for a data set of any size is the difference between a retry
and an afternoon.

With 'resume' set and a target given, the curl and wget methods
continue the partial file. The clean-up in 'Download' skips the
target for the same reason: otherwise it would throw away the very
thing the next attempt is meant to continue.

Methods that cannot resume decline the request rather than
proceeding to avoid destroying the partial file the resuming
methods need.

'resume' is deliberately left unbound when the caller does not ask
for it, rather than defaulted to false.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

The clean-up added in #103 removes 'opt.target' after any failed method,
which also removes a file that was there before the call.  A failed
download then destroys a file it never created.

Remember whether the file existed when 'Download' was entered, and remove
it only if it did not.

This also lets the wget method drop its own removal: with the caller's
file protected in one place, the per-method clean-up is redundant, and
'RemoveFile' is now called from exactly one place instead of two that
disagreed about whether to signal an error when it fails.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>