Better error message when container configuration cannot be read

If an error occurs while trying to read the active container configuration, the user gets the following message:

Could not read active container configuration: null

The "null" appears if there is no exception message, such as when a NullPointerException is thrown.

We should probably be more specific about what is going on here, rather than relying on the error message. We can begin to address this issue by creating a test that enters this branch to explore the failure scenario.

Relevant method: org.jboss.arquillian.impl.client.container.ContainerRegistryCreator.getActivatedConfiguration