Do not ignore named_server_dumpdb() result code

Do not ignore named_server_dumpdb() result code

The named_server_dumpdb() function, which is called when a 'rndc dumpdb'
command is issued, returns a 'isc_result_t' result code and it has been
always ignored since its introduction in
eb8713ed947fdf22a41dad673d561896dd6fe4a2, where it was still called
ns_server_dumpdb(). The orignal reasoning is not preserved, but it could
have been also a simple copy-paste mistake, as there are commands, which
return 'void' and require manually setting 'result = ISC_R_SUCCESS;', as
it was done here. Anyway, named will now return the actual result, and
'rndc' will report an error, when the 'dumpdb' command fails.