handle async route errors

[nikku]

Related to #487

[nikku]

This improves the situation described in #487, as it makes sure that errors in async routes are actually caught. It only really improves direct access to the .json file though.

If the file is consumed via this.fetch during preloading it will now complain about fetch returning invalid JSON. It returns the actual error text.

FetchError: invalid json response body at http://127.0.0.1:3000/index.json reason: Unexpected token O in JSON at position 0
    at eval (webpack:///./__sapper__/server.js?:810:32)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

[Rich-Harris]

nice, thank you!