SPRTS-101: prevent subrack from staying in UNKNOWN when polling fails

There is a bug in the PollingComponentManager where if the polled component fails a poll, the Tango device goes into UNKNOWN state, but never comes out of it once the poll succeeds again.

See ska-telescope/ska-tango-base!133 for details - I suspect this won't be merged quickly, but in the meantime this is hurting us in AAVS3 due to the error described in SPRTS-101.