core/device: demote "strictly unamanged" error
The "device ... not available because device is strictly unmanaged" is almost certainly the least interesting of the reasons why connection can't be activated on a device.
Invent a new error level for it and demote it.
Before:
Error: Connection activation failed: No suitable device found for this connection (device lo not available because device is strictly unmanaged).
After
Error: Connection activation failed: No suitable device found for this connection (device eth0 not available because profile is not compatible with device (...)).