cat-file: handle streaming failures consistently

There are three ways to convince cat-file to stream a blob:

  - cat-file -p $blob

  - cat-file blob $blob

  - echo $batch | cat-file --batch

In the first two, we simply exit with the error code of
streaw_blob_to_fd(). That means that an error will cause us
to exit with "-1" (which we try to avoid) without printing
any kind of error message (which is confusing to the user).

Instead, let's match the third case, which calls die() on an
error. Unfortunately we cannot be more specific, as
stream_blob_to_fd() does not tell us whether the problem was
on reading (e.g., a corrupt object) or on writing (e.g.,
ENOSPC). That might be an opportunity for future work, but
for now we will at least exit with a sane message and exit
code.

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

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