knitr and stopifnot replacement.

I have a daily generated report in which I put a check using stopifnot.  Unfortunately
I didn?t check the effect very well, turns out that if the condition checked fails
this is not shown in the knitr output (I only get an error much later due to a missing
object).

So my question is, what is the correct way to use assertions in a <filename>.Rnw
file.  I want to perform a check, and if it fails everything should stop with a message
I can generate.

Thanks,

Best,
Bart