Throw an error if inline result is not coercible to character

[cderv]

This fix #2006

• any inline result will be coerced to character by as.character()
• If this result in an error, we catch it a thow our own error with a helpful message.

I wanted to indicate the line in the error message but it seems for inline code we don't actually get the location in the file I expected. So I just added the inline code that errors in the message. It is a start.

With this example

---
output:
  html_document: default
---

Sentence with inline `r function() "function"`.

from the original issue in rmarkdown, we now get

> rmarkdown::render("test.Rmd")


processing file: test.Rmd
  |.............................................................................................| 100%
   inline R code fragments

Quitting from lines 2-9 (test.Rmd) 
Error: Result is not coercible to character in this inline code: 'function() "function"'

Note that the lines indicated are not so useful because it is the all files. Contrary to code block, inline code are not exactly located on a line number.

Ideas from improvement for inline code chunk:

Quitting from lines 6 (test.Rmd) 
Error: Result is not coercible to character in inline code line 6: 'function() "function"'

First line above is from all code proccessing (block an inline).
Second block is from inline execution only.

Anyway, this could be for another time

Also, I added unit test for this and a NEWS item as Major Changes as potential breaking change

[yihui]

Thanks!

[cderv]

In fact knitr-example is failing but not sure this is related... 🤔
https://github.com/yihui/knitr/runs/2695278091#step:11:1711

Do you see what would be the cause ? Is it an external change ?

Maybe I should schedule a weekly run on knitr-examples repo so that we know if the master branch break due to external change.

[yihui]

Do you see what would be the cause ? Is it an external change ?

Yes, it's because of a change in formatR: https://github.com/yihui/formatR/releases/tag/v1.10 Comments that are wrapped may become narrower than before. I just committed the change to knitr-examples.

Maybe I should schedule a weekly run on knitr-examples repo so that we know if the master branch break due to external change.

That sounds good to me.