submodule.c: report the submodule that an error occurs in

submodule.c: report the submodule that an error occurs in

When an error occurs in updating the working tree of a submodule in
submodule_move_head, tell the user which submodule the error occurred in.

The call to read-tree contains a super-prefix, such that the read-tree
will correctly report any path related issues, but some error messages
do not contain a path, for example:

  ~/gerrit$ git checkout --recurse-submodules origin/master
  ~/gerrit$ fatal: failed to unpack tree object 07672f31880ba80300b38492df9d0acfcd6ee00a

Give the hint which submodule has a problem.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>