Fix #24: improve error messages during compilation

My goal with the added `echo` statements is to make it easier for people with custom buildpacks to debug the `detect` script. Hopefully that also solves issue #24.
So what should the code be? Like I said [above](https://github.com/ddollar/heroku-buildpack-multi/pull/25/files#r9174816), Bash is not my strong suit.