render_rmarkdown() should surface R Markdown errors better

[maelle]

I think there's a bug somewhere, see the non informative errors encountered in #2168 and #2166

[maelle]

the only error one gets is something à la

Error in `$<-.data.frame`(`*tmp*`, "call_text", value = c("pkgdown::deploy_to_branch(new_process = FALSE)",  : 
  replacement has 15 rows, data has 14
Calls: <Anonymous> ... trace_format -> trace_as_tree -> $<- -> $<-.data.frame

[joelnitta]

+1: I have also just encountered this, and even though a similar cryptic warning as above was issued, the actual problem (missing library() call for a particular package) had nothing to do with a data frame, and was completely different from the cause of the problem in #2166.

[hadley]

This was a bug in rlang that's now fixed. You can update rlang to get the real error.