FailsafeFuture.completeExceptionally should be synchronized

Author: jhalterman

src/main/java/net/jodah/failsafe/FailsafeFuture.java

@@ -50,7 +50,7 @@ public synchronized boolean complete(T value) {
    * handlers.
    */
   @Override
-  public boolean completeExceptionally(Throwable failure) {
+  public synchronized boolean completeExceptionally(Throwable failure) {
     return completeResult(ExecutionResult.failure(failure));
   }