Improve error message when associations are not defined.

src/ORM/Table.php

@@ -968,7 +968,7 @@ public function getAssociation($name)
     {
         $association = $this->findAssociation($name);
         if (!$association) {
-            throw new InvalidArgumentException('Association does not exist: ' . $name);
+            throw new InvalidArgumentException("The {$name} association is not defined on {$this->getAlias()}.");
         }
 
         return $association;