When online_data_migrations raise exceptions, nova/cinder-manage catches
the exceptions, prints fairly useless "something didn't work" messages,
and moves on. Two issues:
1) The user(/admin) has no way to see what actually failed (exception
detail is not logged)
2) The command returns exit status 0, as if all possible migrations have
been completed successfully - this can cause failures to get missed,
especially if automated
This change adds logging of the exceptions, and introduces a new exit
status of 2, which indicates that no updates took effect in the last
batch attempt, but some are (still) failing, which requires intervention.
Change-Id: Ib684091af0b19e62396f6becc78c656c49a60504
Closes-Bug: #1796192
3eea37b85b