Added assertion message to testDns failures

Identify tests that fail testDns because of no network connection.

Bug:26560000
Change-Id: I7683d54d2932ec46c5850bf360c602f6548b179e
diff --git a/tests/cts/net/src/android/net/cts/DnsTest.java b/tests/cts/net/src/android/net/cts/DnsTest.java
index 0377d04..7d485a6 100644
--- a/tests/cts/net/src/android/net/cts/DnsTest.java
+++ b/tests/cts/net/src/android/net/cts/DnsTest.java
@@ -62,7 +62,8 @@
         try {
             addrs = InetAddress.getAllByName("www.google.com");
         } catch (UnknownHostException e) {}
-        assertTrue(addrs.length != 0);
+        assertTrue("[RERUN] DNS could not resolve www.gooogle.com. Check internet connection",
+            addrs.length != 0);
         boolean foundV4 = false, foundV6 = false;
         for (InetAddress addr : addrs) {
             if (addr instanceof Inet4Address) foundV4 = true;