Using `skip_if_offline()` when curl is not installed results in a confusing error

[paleolimbot]

After #1866 , we get the following in Arrow's CI: https://github.com/apache/arrow/actions/runs/6455470463/job/17523978571?pr=38002#step:10:23987

-- Error ('test-parquet.R:485:3'): Can read Parquet files from a URL -----------
Error in `paste0("The ", pkg, " package must be installed in order to use `", 
    fun, "`")`: argument "fun" is missing, with no default

I imagine this was not the intended error message for this case!

[paleolimbot]

For reference, the Arrow issue was apache/arrow#38138 (which we fixed by installing curl, as we should have all along!)

[hadley]

Doh