[9.20] 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 (closed)
Backport of MR !9547 (merged)