DOCUMENTATION: Add vignette on how to troubleshoot

[HenrikBengtsson]

Add vignette on how to troubleshoot. It should explain things like:

• how and when errors are signaled
• how to use plan(transparent) (which uses early signaling and non-local evaluation)
• how to get a traceback

[HenrikBengtsson]

Comment: In future (>= 1.9.0), standard output will be relayed when the value of a future is queried. This will also take place when there's an error. Because of this, it will be possible to do poor man's debugging by adding debug output (stdout only) to narrow down why/where in the code there error occurs. For example,

> library(future.apply)
> x <- as.list(1:4)
> x[[3]] <- "b"
> y <- future_lapply(x, FUN = function(x) { str(x); log(x) })
 int 1
 int 2
 chr "b"
Error in log(x) : non-numeric argument to mathematical function
>