delete_refs(): make error message more generic

Change the error message from Could not remove branch %s to could not remove reference %s First of all, the old error message referred to "branch refs/remotes/origin/foo", which was awkward even for the existing caller. Normally we would refer to a reference like that as either "remote-tracking branch origin/foo" or "reference refs/remotes/origin/foo". Here I take the lazier alternative. Moreover, now that this function is part of the refs API, it might be called for refs that are neither branches nor remote-tracking branches. While we're at it, convert the error message to lower case, as per our usual convention. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>