http-fetch.c: consolidate code to detect missing fetch target

http-fetch.c: consolidate code to detect missing fetch target

At a handful places we check two error codes from curl library
to see if the file we asked was missing from the remote (e.g.
we asked for a loose object when it is in a pack) to decide what
to do next.  This consolidates the check into a single function.

NOTE: the original did not check for HTTP_RETURNED_ERROR when
error code is 404, but this version does to make sure 404 is
from HTTP and not some other protcol.

Signed-off-by: Junio C Hamano <junkio@cox.net>