Wrap glue errors, if possible

[hadley]

Fixes #229

[jennybc]

This doesn't seem to be the error we're trying to test.

After staring long and hard at the CI failures, I realized this. I was puzzled why the R version mattered, since we're using rlang::abort(). But this error is coming from base R.

[jennybc]

I think we either have to give identity_transformer(envir =) a default value or specify it explicitly in the tests that call it directly. envir = parent.frame() feels reasonable to me and consistent with other functions in glue. But I'm not wed to this solution.

[hadley]

Yeah, that looks right to me. Not sure how I missed it originally.