if `offline=FALSE`, better indicator that the iframes won't render in the rstudio web viewer

[chriddyp]

if offline is disabled, it's confusing to see a blank screen when you runApp

A couple options:

• Is it possible to open the app in the web browser instead of the rstudio viewer if offline=FALSE?
• Maybe show a "Unfortunately, IFrames like this plotly graph can't be viewed in RStudio. View this app in your web browser instead" placeholder instead of the blank iframe. If the onerror event is fired from shiny, we could show the placeholder on that event: http://stackoverflow.com/questions/15273042/catch-error-if-iframe-src-fails-to-load-error-refused-to-display-http-ww

[cpsievert]

Good point. Unfortunately, it appears that the RStudio viewer blocks alert() messages as well. One (easy) solution would be relay a message to the R console, but that might easily be missed (especially if the app spits output to the console). Maybe the more robust solution is to insert a div when the onerror message fires?