HTTP/2 SimplePromiseAggregator tryFailure not consistent with setFailure

Motivation:
The SimplePromiseAggregator.setFailure allows a failure to occur before newPromise is called, but tryFailure doesn't. These methods should be consistent.

Modifications:
- tryFailure should use the same logic as setFailure

Result:
Consistent failure routines.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2CodecUtil.java

@@ -240,7 +240,7 @@ public final class Http2CodecUtil {
 
         @Override
         public boolean tryFailure(Throwable cause) {
-            if (awaitingPromises()) {
+            if (allowFailure()) {
                 ++failureCount;
                 if (failureCount == 1) {
                     promise.tryFailure(cause);
@@ -261,7 +261,7 @@ public final class Http2CodecUtil {
          */
         @Override
         public ChannelPromise setFailure(Throwable cause) {
-            if (awaitingPromises() || expectedCount == 0) {
+            if (allowFailure()) {
                 ++failureCount;
                 if (failureCount == 1) {
                     promise.setFailure(cause);
@@ -271,6 +271,10 @@ public final class Http2CodecUtil {
             return this;
         }
 
+        private boolean allowFailure() {
+            return awaitingPromises() || expectedCount == 0;
+        }
+
         private boolean awaitingPromises() {
             return successfulCount + failureCount < expectedCount;
         }