branch: mention start_name in set-upstream error messages

branch: mention start_name in set-upstream error messages

If we refuse a branch operation because the tracking
start_name the user gave us is bogus, we just print
something like:

 fatal: Cannot setup tracking information; start point is not a branch

If we mention the actual name we tried to use, that may help
the user figure out why it didn't work (e.g., if they gave
us the arguments in the wrong order).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>