http-fetch: nicer warning for a server with unreliable 404 status

When a repository otherwise properly prepared is served by a
dumb HTTP server that sends "No such page" output with 200
status for human consumption to a request for a page that does
not exist, the users will get an alarming "File X corrupt" error
message.  Hint that they might be dealing with such a server at
the end and suggest running fsck-objects to check if the result
is OK (the pack-fallback code does the right thing in this case
so unless a loose object file was actually corrupt the result
should check OK).

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