Improve error message for issue with inline expression evaluation

[cderv]

From rstudio/bookdown#1345

Current error looks like this

knitr::knit(text = "`r = 0.06`")
#> Quitting from lines 1-1 () 
#> Error in parse(text = code, keep.source = FALSE): <text>:1:1: '=' inattendu(e)
#> 1: =
#>     ^

Would be interesting to catch the issue specifically and then fail with a more helpful message on the cause.

Error message is from here: yihui/knitr@907184f/R/hooks.R#L14

[github-actions]

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