* linstor: Add util method getBestErrorMessage from main
* linstor: failed remove of allow-two-primaries is no fatal error
* linstor: Fix failure if a Linstor node is down while migrating
If a Linstor node is down while migrating resource, allow-two-primaries
setting will fail because we can't reach the downed node. But it will
still set the property on the other nodes and migration should work.
We now just report an error instead of completely failing.