Better error messaging when using execpath and other make expansions

[alexeagle]

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?