Capture error in eng_sql if error = TRUE

[cderv]

This will fix rstudio/rmarkdown#1208

We capture the error of SQL execution if error = TRUE as for some other engine.

This allows this Rmd to work now without stoping

---
title: "Error Honored"
output: html_document
---

```{r, message=FALSE}
knitr::opts_chunk$set(error = TRUE)
library(DBI)
con <- dbConnect(RSQLite::SQLite(), ":memory:")
```

```{r}
stop('error message that I want to print')
```

```{r}
dbWriteTable(con, "mtcars", mtcars)
```

```{sql, connection=con}
SELECT * FROM mtcars WHERE cyl = 4
```

```{sql, connection=con}
DROP TABLE mtcars
```

```{r}
dbListTables(con)
```

```{sql, connection=con}
select test;
```

I'll do a PR in knitr-example to complement this PR with tests

[yihui]

Thanks!

[cderv]

One thought after having pushed that...

How does that work in knitr so that the error hook is used ? For example if someone want to use a specific class.error, I am not sure what I have done is enough. More generally, I don't think for non R engine that wrap.error will be called correctly.
Should this be improve ?

[yihui]

This could be improved. The out argument of engine_output() can take a list. You could add the error message to the list, and make its class to be error, then knitr:::wrap should handle it.

The work is probably not trivial, so I'd rather wait for someone to request it before we spend the time on the improvement.

[cderv]

Thanks.

Let's wait then.