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.

[Nutomic]

This is much better, thanks! However the line number in the error message is wrong. It always points to the line where the view! macro definition starts, not where inside the macro the error occurs. In my case it always reports an error at src/frontend/components/nav.rs:22:5 which is here, even after adding more lines at the beginning of view!

[gbj]

I am aware that it currently returns the line number of the view macro, rather than the element.

[gbj]

But note that it should also log out the actual DOM node, which you can right-click on to go to its location in the DOM inspector -- which should help narrow it much more than the view line number.

[gbj]

@Nutomic See #3293 for improved error location: now the line/column should point to the actual element in the view, not the view macro as a whole.