Update path to pathfinder (and fix error message)

`erikvold/addon-pathfinder` for whatever reason now points to `erikvold/addon-sdk`, and the repo we're looking for is at `erikvoid/pathfinder`. (To add to the confusion, `erikvold/addon-sdk` has a pathfinder branch)

Also fix the build.sh error message to check for both repos and point you to bootstrap.sh or README.

(Ideally these should be submodules)