Errors should no longer be cleared

[guybedford]

We should now seek to follow the error behaviour of module scripts, as detailed in whatwg/html#2595.

In short:

• Retain the error state in the registry indefinitely
• Abort the instantiation process if there is a load error
• Rethrow only at the execution phase if there is an execution error.

[guybedford]

This has been implemented, with support for System.delete to clear this state (even though it isn't visible in the public registry.

[guybedford]

Released in 2.2.0.