More sane handling of errors during failover

Currently we ignore errors on instance shutdown (on the source node)
during instance failover. We should do this only if the user gave a
command line options allowing this, as it's a dangerous thing to do.

This patch fixes this by using the same "--ignore-consistency" option
for deciding whether to continue or abort. It also expands a bit the man
page.

Reviewed-by: imsnah