Remove old, problematic, and unnecessary renderError and clearError method defaults

[cpsievert]

Closes #490

By removing these defaults, when the widget is in rendered in Shiny, it will simply fallback to Shiny's OutputBinding default methods, which does a better and simpler job of rendering and clearing errors.

And, when rendering statically, the renderError/clearError methods aren't relevant

[cpsievert]

After looking at this again, I now realize there is an intentional difference here and the generic output binding renderError. In the widget case, when an error gets rendered after a widget has been rendered, the widget container's height doesn't change, meaning there is no reflow of content below it. This isn't the case for something like uiOutput().

We could still move away from the setTimeout() to ResizeObserver(), but that's another matter.