Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/bcrypt-pbkdf/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing
+
+This repository uses [cr.joyent.us](https://cr.joyent.us) (Gerrit) for new
+changes. Anyone can submit changes. To get started, see the [cr.joyent.us user
+guide](https://github.com/joyent/joyent-gerrit/blob/master/docs/user/README.md).
+This repo does not use GitHub pull requests.
+
+See the [Joyent Engineering
+Guidelines](https://github.com/joyent/eng/blob/master/docs/index.md) for general
+best practices expected in this repository.
+
+If you're changing something non-trivial or user-facing, you may want to submit
+an issue first.