patman: make "No recipient" checking more tolerant

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 = [email protected]

in their .git/config because the 'To: [email protected]' field
should always be added when sending patches.

That seems more reasonable rather than adding
'Series-to: [email protected]' to every patch series.

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

Signed-off-by: Masahiro Yamada <[email protected]>
Cc: Simon Glass <[email protected]>
Acked-by: Simon Glass <[email protected]>