Cleanup for --skip-content flag

[danielbachhuber]

• Cleanup tests for greater specificity.
• Move errors to an earlier position to prevent unnecessary code execution.
• Use the appropriate cache key name for --skip-content.
• Avoid duplicate API requests by executing code path earlier.

Fixes #36

[danielbachhuber]

@AaronRutley ^ Worth looking through those changes to see the cleanup I did.

[AaronRutley]

@danielbachhuber! I've just had a look over your cleanup commits, it all makes sense, thanks!