More graceful error message (or no error message) when running an app with a call to runApp() in it

[daattali]

Every time I give a shiny lecture, I get a few students trying to place runApp() inside their source code after seeing it happen when they click the "Run App" button in RStudio. The error message you get is

Warning: Error in handlers$add: Key / already in use
Stack trace (innermost first):
    44: handlers$add
    43: handlerManager$addHandler
    42: startApp
    41: runApp
     1: runApp
Error : Key / already in use

Not very informative. It'd be nice to get a better message of maybe even somehow be able to handle the case of having runApp() in the code

[wch]

This is the same as #1358.