[FR] Fail early if `.bib` file is not found

[nanxstats]

Minimal reproducible example:

---
title: "Untitled"
output: html_document
bibliography: references.bib
---

```{r}
# Long compute
Sys.sleep(3600)
```

Save as zzz.Rmd. Running rmarkdown::render("zzz.Rmd") will fail after an hour with all compute finished but no output file generated, with the following error, simply because the .bib file is not there:

File references.bib not found in resource path
Error: pandoc document conversion failed with error 99

I wonder if it makes sense to fail early here. It will protect the users from waiting for a long time to get no results due to a trivial condition that can be easily verified by rmarkdown.

[yihui]

I think that's a good idea. The output_file/output_dir/intermediates_dir business in render() may once again complicates this seemingly simple issue, though (i.e., the pain of multiple possible working directories).