Fix unused variable warning

```
lib/async/pool/controller.rb:332: warning: assigned but unused variable - error
```

Author: byroot

lib/async/pool/controller.rb

@@ -328,7 +328,7 @@ def available_resource
 				end
 
 				return resource
-			rescue Exception => error
+			rescue Exception
 				reuse(resource) if resource
 				raise
 			end