box: prevent infinite elections after promote()

When a node has Raft turned on, issuing `box.ctl.promote()` on it triggers
Raft elections. Such promotion bypasses the Pre-Vote check, because we
want manual promotion to succeed even if it disrupts the current leader.

However, in order to win elections, the node still has to collect votes
from a quorum of peers. When there's no quorum, promote() might enter an
infinite election loop, trying to promote the current node.

Let's not start elections when it's known for sure that the node won't
be able to win them, and make promote() throw an appropriate error.

Now the node still triggers at least one election round on
box.ctl.promote(), but it doesn't attempt re-running elections
infinitely when there is no quorum.

Follow-up #6654

NO_DOC=minor change