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/wrappy/README.md

@@ -0,0 +1,36 @@
+# wrappy
+
+Callback wrapping utility
+
+## USAGE
+
+```javascript
+var wrappy = require("wrappy")
+
+// var wrapper = wrappy(wrapperFunction)
+
+// make sure a cb is called only once
+// See also: http://npm.im/once for this specific use case
+var once = wrappy(function (cb) {
+  var called = false
+  return function () {
+    if (called) return
+    called = true
+    return cb.apply(this, arguments)
+  }
+})
+
+function printBoo () {
+  console.log('boo')
+}
+// has some rando property
+printBoo.iAmBooPrinter = true
+
+var onlyPrintOnce = once(printBoo)
+
+onlyPrintOnce() // prints 'boo'
+onlyPrintOnce() // does nothing
+
+// random property is retained!
+assert.equal(onlyPrintOnce.iAmBooPrinter, true)
+```