Propagating of errors from the Apiary.io service

Closes #20.

Those `unless (200..299).include? response.code` statements would never get to be executed anyway, because `RestClient` raises exceptions in case the response code isn't okay.

Not sure if there's documentation to the Apiary.io service - I just inspected the responses and tried to get the best from them (unfortunately, no line/character info is available, so those new error messages are not actually very helpful anyway for debugging the blueprint).

If preview server is running, it logs the stacktrace of `abort` statements and continues to run. I am not sure what is the best solution of that - I thought it should just shut down with the error message, but I couldn't get to that behaviour. I kept using `abort`s so it follows the convention I've seen in everywhere else the gem.

The code's not very DRY, but I feel still quite new to Ruby so I could refactor it into something more elegant.