Implement exceptions.PreventUpdate

[cpsievert]

plotly/dash#190

plotly/dash@fd2f353
plotly/dash@ad92fa3

[rpkyle]

@nicolaskruchten @alexcjohnson I was talking with @KPhans not that long ago, and he asked whether this was on the list of features to support. We haven't discussed this item specifically.

It exists in Dash for Python, so I am assuming that it is?

[alexcjohnson]

Definitely should be on the list. Also see dash.no_update plotly/dash#680 which is only strictly necessary when we get to multi outputs but is a nice alternative to raising an error even for single outputs.

[rpkyle]

Definitely should be on the list. Also see dash.no_update plotly/dash#680 which is only strictly necessary when we get to multi outputs but is a nice alternative to raising an error even for single outputs.

We discussed the implementation of PreventUpdate in R, and I believe the initial plan is to use something like no_update. I'm working on that today, will attempt to draft a PR by tomorrow.