nfsd.c: Log a more meaningful failure message

For the cases where the nfsd(8) daemon is already running or
has failed to start within a prison due to an incorrect prison
configuration, the failure message logged is:

Can't read stable storage file: operation not permitted

This patch replaces the above with more meaningful messages.
It depends on commit 10dff9da9748 to differentiate between the
above two cases, however even without this commit, the messages
should be an improvement.

(cherry picked from commit fe5c211ba87315b098769a7e6d50b41f00f6fed4)