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/arr-diff/index.js

@@ -0,0 +1,47 @@
+/*!
+ * arr-diff <https://github.com/jonschlinkert/arr-diff>
+ *
+ * Copyright (c) 2014-2017, Jon Schlinkert.
+ * Released under the MIT License.
+ */
+
+'use strict';
+
+module.exports = function diff(arr/*, arrays*/) {
+  var len = arguments.length;
+  var idx = 0;
+  while (++idx < len) {
+    arr = diffArray(arr, arguments[idx]);
+  }
+  return arr;
+};
+
+function diffArray(one, two) {
+  if (!Array.isArray(two)) {
+    return one.slice();
+  }
+
+  var tlen = two.length
+  var olen = one.length;
+  var idx = -1;
+  var arr = [];
+
+  while (++idx < olen) {
+    var ele = one[idx];
+
+    var hasEle = false;
+    for (var i = 0; i < tlen; i++) {
+      var val = two[i];
+
+      if (ele === val) {
+        hasEle = true;
+        break;
+      }
+    }
+
+    if (hasEle === false) {
+      arr.push(ele);
+    }
+  }
+  return arr;
+}