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/lodash/_baseCreate.js

@@ -0,0 +1,30 @@
+var isObject = require('./isObject');
+
+/** Built-in value references. */
+var objectCreate = Object.create;
+
+/**
+ * The base implementation of `_.create` without support for assigning
+ * properties to the created object.
+ *
+ * @private
+ * @param {Object} proto The object to inherit from.
+ * @returns {Object} Returns the new object.
+ */
+var baseCreate = (function() {
+  function object() {}
+  return function(proto) {
+    if (!isObject(proto)) {
+      return {};
+    }
+    if (objectCreate) {
+      return objectCreate(proto);
+    }
+    object.prototype = proto;
+    var result = new object;
+    object.prototype = undefined;
+    return result;
+  };
+}());
+
+module.exports = baseCreate;