send-email: error out when relogin delay is missing

send-email: error out when relogin delay is missing

When the batch size is neither configured nor given on the command
line, but the relogin delay is given, then the current code ignores
the relogin delay setting.

This is unsafe as there was some intention when setting the batch size.
One workaround would be to just assume a batch size of 1 as a default.
This however may be bad UX, as then the user may wonder why it is sending
slowly without apparent batching.

Error out for now instead of potentially confusing the user.
As 5453b83bdf (send-email: --batch-size to work around some SMTP
server limit, 2017-05-21) lays out, we rather want to not have this
interface anyway and would rather want to react on the server throttling
dynamically.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>