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/array-equal/index.js

@@ -0,0 +1,9 @@
+
+module.exports = function equal(arr1, arr2) {
+  var length = arr1.length
+  if (length !== arr2.length) return false
+  for (var i = 0; i < length; i++)
+    if (arr1[i] !== arr2[i])
+      return false
+  return true
+}