When turning DOM exceptions into `ErrorInfo` always try to stringify
the JavaScript value, even if it's an object that isn't a `DOMException`
or native exception. This means that exceptions that extend the `Error`
prototype are now stringified. The result is that test output for WPT
global assertion failures is more useful. For instance for the test
include-frames-from-child-same-origin-grandchild.sub.html:
Before:
```
uncaught exception: unknown (can't convert to string)
```
After:
```
uncaught exception: Error: assert_equals: expected 4 but got 3
```