sha1-file: avoid "sha1 file" for generic use in messages

These error messages say "sha1 file", which is vague and not common in
user-facing documentation. Unlike the conversions from the previous
commit, these do not always refer to loose objects.

In finalize_object_file() we could be dealing with a packfile. Let's
just say "unable to write file" instead; since we include the filename,
the nature of the file is clear from the rest of the message.

In force_object_loose(), we're calling into read_object(), which could
actually be _any_ type of object. Just say "object".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
