okhttp: Remove wrong status description

The != should have been ==. However, it is provable that the exception
won't be null, but we want to make that fact obvious when auditing. So
we just fail if the exception is ever null.
diff --git a/okhttp/src/main/java/io/grpc/okhttp/OkHttpClientTransport.java b/okhttp/src/main/java/io/grpc/okhttp/OkHttpClientTransport.java
index 7e14730..544851a 100644
--- a/okhttp/src/main/java/io/grpc/okhttp/OkHttpClientTransport.java
+++ b/okhttp/src/main/java/io/grpc/okhttp/OkHttpClientTransport.java
@@ -548,10 +548,8 @@
    * Finish all active streams due to an IOException, then close the transport.
    */
   void onException(Throwable failureCause) {
+    Preconditions.checkNotNull(failureCause, "failureCause");
     Status status = Status.UNAVAILABLE.withCause(failureCause);
-    if (failureCause != null) {
-      status = status.augmentDescription("No provided cause");
-    }
     startGoAway(0, ErrorCode.INTERNAL_ERROR, status);
   }