xopen: explicitly report creation failures

If the flags O_CREAT and O_EXCL are both given then open(2) is supposed to create the file and error out if it already exists. The error message in that case looks like this: fatal: could not open 'foo' for writing: File exists Without further context this is confusing: Why should the existence of the file pose a problem? Isn't that a requirement for writing to it? Add a more specific error message for that case to tell the user that we actually don't expect the file to preexist, so the example becomes: fatal: unable to create 'foo': File exists Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>