output file and line when printing error

Example output:

```
test.Rmd:10-11: Chunk unnamed-chunk-2:
  Error: non-numeric argument to binary operator 


Number of messages:
error 
    1 
```

Input:

    Test.
    
    ```{r echo=F}
    options(knitr.package.verbose=T)
    opts_chunk$set()
    ```
    
    Bar.
    
    ```{r}
    1 + 'a'
    ```
    
    Baz.
    
The erroring chunk is actually three lines long, this one-off error will be handled by a separate PR.

Fixes #688.