docs: adds more helpful text for hydration errors (closes #3267)

[gbj]

Error messages in the case of hydration problems are currently very unhelpful in 0.7, as I had not yet done anything to make them more useful. For example, if the renderer was expected to find an element and found something else instead, you just got an .unwrap() inside HtmlElement::hydrate().

This adds some debugging machinery that attempts to do the following:

1. Provide a useful error message.
2. List the location of the view! or element that is currently being hydrated, to help narrow down problems.
3. Logs out the unexpected node as part of the error message in the JS console. Most browser devtools allow you to right-click and go to where that node is used in the page, which means you can navigate directly to wherever in your application the unexpected node was found.