More sane handling of errors during failover

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