Add the ability to view devices that were identified but failed to be added

One of the best features of Observium is automatic discovery by spidering a network using CDP/LLDP. However, the current implementation will only add a device if the discovered hostname matches a usable DNS name (or local hosts file).

The web-interface needs a way to show admins which devices were identified during discovery, but then failed to be added for some reason. This information may already exist in the "Links" table.

With this information, admins can correct the the problem and Observium will discover the fixed devices during the next discovery.