[Rd] Sweave provides a misleading error when vignette engine not fully specified

[Martin Morgan]

Trying to Stangle / Sweave a file

$ cat vignette.Rnw
%\VignetteEngine{knitr}
\documentclass{article}
\begin{document}
\end{document}

results in a misleading error message:

~/tmp$ R CMD Stangle vignette.Rnw
Error: Vignette engine package not specified
Execution halted

when what is missing is the full specification knitr::knitr; 'vignette engine 
package and function not specified' ? Also it's somehow unfortunate that the 
vignette builds when in a package/vignettes directory, but not as a stand-alone 
document.

Also for what its worth Sweave'ing still fails to produce graphics output

https://stat.ethz.ch/pipermail/r-devel/2014-February/068414.html

$ R --version|head -n 3
R Under development (unstable) (2014-03-05 r65124) -- "Unsuffered Consequences"
Copyright (C) 2014 The R Foundation for Statistical Computing
Platform: x86_64-unknown-linux-gnu (64-bit)

Martin
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