Replace all fatal(), check_result(), and check_dns_dbiterator_current()
calls inside verify_nodes() with zoneverify_log_error() calls and error
handling code. Enable verify_nodes() to signal errors to the caller
using its return value.
Modify the call site of verify_nodes() so that its errors are properly
handled.
Free all heap elements upon verification context cleanup as a
verification error may prevent them from being freed elsewhere.
Remove the check_dns_dbiterator_current() macro as it is no longer used
anywhere in lib/dns/zoneverify.c.
4354f44d9c