Downgrade DNS fail message from Error to Warning

Most DNS fails are caused by network issues, like connection timed out.
Lower log severity from error to warning to conform to go/greenlog.

Test: 1. connect to a WiFi AP without WAN
      2. atest CtsNetTestCases:android.net.cts.DnsResolverTest
      3. adb logcat | grep resNetworkResult:android
Bug: 181269159
Change-Id: Ifeaf250c91a4b9123d200bd62f085c11009f2491