Add a string message for Private DNS strict mode failure

Add a message to be displayed whenever a Private DNS provider
hostname is unusable.  It might be that the name fails to resolve
or that network errors prevent us from connecting -- the string
need give no indication of the specific failure mode, only its
unusability.

Test: treehugger
Bug: 64133961
Bug: 74844869
Change-Id: I361291229241854dc3b9b84407baf500e84a1832