`log` is `nil`, unless we are in verbose mode. Skip logging if no logger
was passed.
Long-term I think we should reconsider how we do our logging, and always
log these type of unexpected errors, and hence always have the logger be
non-nil. For now, fix the immediate problem.
The fact that loading the store from disk fails on upgrade is
unexpected, and should be fixed separately.
Fixes#685.
Signed-off-by: Tom Gundersen <teg@jklm.no>
71b69e5ad2