core: Upgrade DC should not log failure on update process.

When VDSM fails to run the verb upgradeStoragePool,
engine should not print a failure in the audit log but a warning,
since the engine does not fail in the process and should eventually
upgrade the master domain and its storages when it will send SPM start
or when VDSM will get the storage connectivity again.

Change-Id: I17597f74eeaae62c716bf7dd8688f3fee2ca2ab6
Bug-Url: https://bugzilla.redhat.com/950637
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>