Check for IO errors after running a command

Check for IO errors after running a command

This is trying to implement the strict IO error checks that Jim Meyering
suggested, but explicitly limits it to just regular files. If a pipe gets
closed on us, we shouldn't complain about it.

If the subcommand already returned an error, that takes precedence (and we
assume that the subcommand already printed out any relevant messages
relating to it)

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>