send-email: detect invocation errors earlier

send-email: detect invocation errors earlier

We never even look at the command line arguments until after
we have prompted the user for some information. So running
"git send-email" without arguments would prompt for "from"
and "to" headers, only to then die with "No patch files
specified." Instead, let's try to do as much error checking
as possible before getting user input.

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