We can cancel a file overwrite at the last second when closing the
stream by setting a cancelled cancellable. Current code was simply not
closing the stream, but this was not enough as overwriting was happening
anyway (probably when finalizing).
This will allow much safe saving process since we would not be
overwriting a previously sane XCF file when an error occurred (either in
our code or a memory error, or whatnot).
See also discussion in #2565.