Gracefully handle lack of -Yrangepos

[kenbot]

Currently if -Yrangepos is not set, then interpolated args cannot be displayed in the leftmost column of the error table, and the position of the ^ indicator cannot be accurately set.

We should gracefully show something meaningful, such as "$ARG" for the argument names (rather than an empty string), and fixing the ^ at the start of the expression. We could also print a message that suggests that the user add scalacOptions += "-Yrangepos" to build.sbt.