implement error state

[dherman]

• All error conditions need to set the [[Error]] field.
• Implement the .error() method.
• Clear out the [[Error]] field when installing.

[caridy]

Most of the work is now in as part of PR #97, the missing piece is the error propagation, I will work on that this week.

[caridy]

https://github.com/whatwg/loader/commit/215ca0e7d092d02503aa1520d2032f5b47f84997