http-push: clarify the reason of error from the initial PROPFIND request

http-push: clarify the reason of error from the initial PROPFIND request

The first thing http-push does is a PROPFIND to see if the other
end supports locking.  The failure message we give is always
reported as "no DAV locking support at the remote repository",
regardless of the reason why we ended up not finding the locking
support on the other end.

This moves the code to report "no DAV locking support" down the
codepath so that the message is issued only when we successfully
get a response to PROPFIND and the other end say it does not
support locking.  Other failures, such as connectivity glitches
and credential mismatches, have their own error message issued
and we will not issue "no DAV locking" error (we do not even
know if the remote end supports it).

Signed-off-by: Junio C Hamano <gitster@pobox.com>