Resolver: added missing event handling after reading.

Resolver: added missing event handling after reading.

If we need to be notified about further events, ngx_handle_read_event()
needs to be called after a read event is processed.  Without this,
an event can be removed from the kernel and won't be reported again,
notably when using oneshot event methods, such as eventport on Solaris.

While here, error handling is also added, similar to one present in
ngx_resolver_tcp_read().  This is not expected to make a difference
and mostly added for consistency.