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/_baseKeysIn.js

@@ -0,0 +1,33 @@
+var isObject = require('./isObject'),
+    isPrototype = require('./_isPrototype'),
+    nativeKeysIn = require('./_nativeKeysIn');
+
+/** Used for built-in method references. */
+var objectProto = Object.prototype;
+
+/** Used to check objects for own properties. */
+var hasOwnProperty = objectProto.hasOwnProperty;
+
+/**
+ * The base implementation of `_.keysIn` which doesn't treat sparse arrays as dense.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Array} Returns the array of property names.
+ */
+function baseKeysIn(object) {
+  if (!isObject(object)) {
+    return nativeKeysIn(object);
+  }
+  var isProto = isPrototype(object),
+      result = [];
+
+  for (var key in object) {
+    if (!(key == 'constructor' && (isProto || !hasOwnProperty.call(object, key)))) {
+      result.push(key);
+    }
+  }
+  return result;
+}
+
+module.exports = baseKeysIn;