code: @Override
  	public void setException(@NotNull Throwable e) {
  		assert !isComplete();
  		exception = e;
  		completeExceptionally(e);
  	}
docs: Sets exception and completes this {@code SettablePromise} exceptionally.
  {@code AssertionError} is thrown when you try to set exception for
  an already completed {@code Promise}.
  
  @param e exception