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-union/index.js

@@ -0,0 +1,29 @@
+'use strict';
+
+module.exports = function union(init) {
+  if (!Array.isArray(init)) {
+    throw new TypeError('arr-union expects the first argument to be an array.');
+  }
+
+  var len = arguments.length;
+  var i = 0;
+
+  while (++i < len) {
+    var arg = arguments[i];
+    if (!arg) continue;
+
+    if (!Array.isArray(arg)) {
+      arg = [arg];
+    }
+
+    for (var j = 0; j < arg.length; j++) {
+      var ele = arg[j];
+
+      if (init.indexOf(ele) >= 0) {
+        continue;
+      }
+      init.push(ele);
+    }
+  }
+  return init;
+};