Issue #206 Throw an exception if the bag is closed, this will cause addConnection() to cleanly close the opened connection and correctly decrement the pool count.

hikaricp-java6/src/main/java/com/zaxxer/hikari/util/ConcurrentBag.java

@@ -179,7 +179,7 @@ public final class ConcurrentBag<T extends BagEntry>
    public void add(final T bagEntry)
    {
       if (closed) {
-         LOGGER.warn("ConcurrentBag has been closed, ignoring add()");
+         throw new IllegalStateException("ConcurrentBag has been closed, ignoring add()");
       }
       else {
          sharedList.add(bagEntry);

hikaricp/src/main/java/com/zaxxer/hikari/util/ConcurrentBag.java

@@ -178,7 +178,7 @@ public final class ConcurrentBag<T extends BagEntry>
    public void add(final T bagEntry)
    {
       if (closed) {
-         LOGGER.warn("ConcurrentBag has been closed, ignoring add()");
+         throw new IllegalStateException("ConcurrentBag has been closed, ignoring add()");
       }
       else {
          sharedList.add(bagEntry);