index-pack: distinguish missing objects from type errors

index-pack: distinguish missing objects from type errors

When we fetch a pack that does not contain an object we
expected to receive, we get an error like:

  $ git init --bare tmp.git && cd tmp.git
  $ git fetch ../parent.git
  [...]
  error: Could not read 964953ec7bcc0245cb1d0db4095455edd21a2f2e
  fatal: Failed to traverse parents of commit b8247b40caf6704fe52736cdece6d6aae87471aa
  error: ../parent.git did not send all necessary objects

This comes from the check_everything_connected rev-list. If
we try cloning the same repo (rather than a fetch), we end
up using index-pack's --check-self-contained-and-connected
option instead, which produces output like:

  $ git clone --no-local --bare parent.git tmp.git
  [...]
  fatal: object of unexpected type
  fatal: index-pack failed

Not only is the sha1 missing, but it's a misleading message.
There's no type problem, but rather a missing object
problem; we don't notice the difference because we simply
compare OBJ_BAD != OBJ_BLOB.  Let's provide a different
message for this case:

  $ git clone --no-local --bare parent.git tmp.git
  fatal: did not receive expected object 6b00a8c61ed379d5f925a72c1987c9c52129d364
  fatal: index-pack failed

While we're at it, let's also improve a true type mismatch
error to look like

  fatal: object 6b00a8c61ed379d5f925a72c1987c9c52129d364: expected type blob, got tree

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