Better error messaging when using execpath and other make expansions

https://github.com/aspect-build/rules_js/pull/108 is an example of user error which is understandable, since rules_js sets the working directory differently from typical Bazel actions. We could:
- detect use of `$(execpath)` etc. without `../` and warn ahead-of-time
- same but only print after the action fails?
- provide different expansion helpers?
- modify behavior of them?

Start with the user-error example in pull request 108 and examine how rules_js handles Bazel make expansions such as $(execpath). The issue presents several unresolved approaches, so done requires choosing and implementing one clear warning, expansion-helper, or behavior change.

Written by the indexing model from the issue text.