There have been reports (rhbz#1851507) where generating the fencing
parameters fails because of one or more servers in error state, like
when a scale out operation doesn't go as planned.
This commit wraps get_by_instance_uuid in a try/except block to prevent
this from happening, skipping the node in error state and allowing
users to still get a valid fencing config for the remaining servers.
Change-Id: I397f8d641504ac2ceed36fa975cf97fac5bbb81a