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/tweetnacl/AUTHORS.md

@@ -0,0 +1,28 @@
+List of TweetNaCl.js authors
+============================
+
+    Alphabetical order by first name.
+    Format: Name (GitHub username or URL)
+
+* AndSDev (@AndSDev)
+* Devi Mandiri (@devi)
+* Dmitry Chestnykh (@dchest)
+
+List of authors of third-party public domain code from which TweetNaCl.js code was derived
+==========================================================================================
+
+[TweetNaCl](http://tweetnacl.cr.yp.to/)
+--------------------------------------
+
+* Bernard van Gastel
+* Daniel J. Bernstein <http://cr.yp.to/djb.html>
+* Peter Schwabe <http://www.cryptojedi.org/users/peter/>
+* Sjaak Smetsers <http://www.cs.ru.nl/~sjakie/>
+* Tanja Lange <http://hyperelliptic.org/tanja>
+* Wesley Janssen
+
+
+[Poly1305-donna](https://github.com/floodyberry/poly1305-donna)
+--------------------------------------------------------------
+
+* Andrew Moon (@floodyberry)