patman: make "No recipient" checking more tolerant

If Series-to tag is missing, Patman exits with a message
"No recipient".

This is just annoying for those who had already added
sendemail.to configuration.

I guess many developers have

  [sendemail]
          to = u-boot@lists.denx.de

in their .git/config because the 'To: u-boot@lists.denx.de' field
should always be added when sending patches.

That seems more reasonable rather than adding
'Series-to: u-boot@lists.denx.de' to every patch series.

Patman should exit only when both Series-to tag and sendemail.to
configuration are mising.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>