Improve error handling if aws.config isn't used correctly

When `aws.config` isn't correctly defined in the `build.xml`, further targets will fail with a somewhat cryptic message like :
```
PHP Fatal error: Uncaught Error: Call to a member function createAutoScaling() on null in vendor/continuousphp/aws-sdk-phing/src/Task/Ec2/AutoScalingGroupCapacityTask.php:121
```
An additional check could perhaps improve the user experience.