Show the whole error stack on load failure

This is in reference to issue #1018

Currently only the original error message is shown. This pull request includes the whole error stack, which immediately points the user to the offending failing line.

Review URL: https://github.com/google/traceur-compiler/pull/1019

Closes #1019.
Fix issue with object literal in the destructuring initializer

When we have

```js
({x = {y: 1}} = expr);
```

we ended up transforming `{y: 1}` as well which leads to an invalid
parse tree.

Fixes #1011