Changes

nft-kpi: log errors of counter retrieval

Make sure errors of getting counters from nft are logged.

Some context: we'll try again each X34 period, hence this is only a
problem when the error persists.

For example, when the get-counters thread is faster than the maintenance
thread can even create the nft table initially, this error will show.

We could add a definite check whether the maintenance thread has created
the tables yet, and wait for that event. But this complexity is not
really needed: it is fine to just fail getting counters once or twice.

Related: SYS#6773
Change-Id: I84340482e4a5bfcac158a21c9378a9511fa5ea10