test.py: Show a specific error if the apiKey is not set

This was why the builds for #5 were failing, not because of the code I changed.

This patch makes it clearer for anyone running `supybot-test` in the future to know that it's the lack of an apikey causing the plugin to fail, *not* their code changes. Without this, the tests just choke with a TypeError as Wunderground doesn't send a reply without a key.