Added ability to prime errors in the cache

Author: aschrijver

src/main/java/io/engagingspaces/vertx/dataloader/DataLoader.java

@@ -188,6 +188,21 @@ public DataLoader<K, V> prime(K key, V value) {
         return this;
     }
 
+    /**
+     * 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
+     */
+    public DataLoader<K, V> prime(K key, Exception error) {
+        Object cacheKey = getCacheKey(key);
+        if (!futureCache.containsKey(cacheKey)) {
+            futureCache.set(cacheKey, Future.failedFuture(error));
+        }
+        return this;
+    }
+
     /**
      * Gets the object that is used in the internal cache map as key, by applying the cache key function to
      * the provided key.