[R] Parser For Line Number Tracing

[Ivo Welch]

I am afraid my errors are worse!  (so are my postings.  I should have
given an example.)

```
x <- 1
y <- 2
nofunction("something stupid I am doing!")
z <- 4
```

and

```
> source("where-is-my-water.R")
Error in nofunction("something stupid I am doing!") :
  could not find function "nofunction"
```

and no traceback is available.