Remove coverage statement errors from KB Sync Error logs

Purpose:
Currently we surface log messages regarding problems with coverage in the 'error' log for each synchronisation. These errors do not stop the resource being ingested and have been identified as unhelpful by users.

We should continue to do the same checks for coverage, but we should only log an error in the user facing error log if the issue will stop the resource from synced

The errors we see of this type typically have the form "Property [startDate] of class [class org.olf.dataimport.erm.CoverageStatement] cannot be null" - these make up the vast majority of errors we currently see in the GOKb sync error log