Fixes #1358: more informative error message when calling runApp inside of an app's app.R

[bborgesr]

Fixes #1358: more informative error message when calling runApp inside of an app's app.R (or inside ui.R or server.R).

This used to be: stop("Key ", key, " already in use")

Now it is: stop("Please remove any calls to runApp from inside your source code. (Details: Key ", key, " already in use)").

The only way I see this may be problematic is if this error is raised in another context (that isn't calling runApp from the app's source code). But I don't think that ever happens (?)

Again, I don't think this merits a NEWS entry... Let me know if you disagree

[wch]

A couple things:

• This message would work better: "Can't call runApp from within runApp. If your application code contains runApp(), please remove it."
• runApp could check this at the top:

  if (.globals$running) {
    stop("HEY!")
  }
  .globals$running <- TRUE
  on.exit({
    .globals$running <- FALSE
  },add = TRUE)

• And .globals$running <- FALSE would have to be already set.