Pixi: consolidate error handling

There are cases in which Pixi might fail during runtime without a exact user error for example when `AMP.getState` calls are failing, APIs returning unexpected responses, `amp-script` computations taking too long and triggering script shutdown, ...

We already handle some of them (sometimes with a simple `console.error`) but we should provide more helpful user feedback for those cases.

See https://github.com/ampproject/amp.dev/pull/4601#pullrequestreview-492583494 for examples.