fix: usr: Fix the 'rndc dumpdb' command's error reporting

The 'rndc dumpdb' command wasn't reporting errors which
occurred when starting up the database dump process by named,
like, for example, a permission denied error for the
'dump-file' file. This has been fixed. Note, however, that
'rndc dumpdb' performs asynchronous writes, so errors can
also occur during the dumping process, which will not be
reported back to 'rndc', but which will still be logged by
named.

Closes #4944

Merge branch '4944-rndc-dumpdb-do-not-ignore-errors' into 'main'

See merge request isc-projects/bind9!9547