code: public DataLoader<K, V> prime(K key, Exception error) {
          Object cacheKey = getCacheKey(key);
          if (!futureCache.containsKey(cacheKey)) {
              futureCache.set(cacheKey, CompletableFutureKit.failedFuture(error));
          }
          return this;
      }
docs: Primes the cache with the given key and error.
  
  @param key   the key
  @param error the exception to prime instead of a value
  
  @return the data loader for fluent coding