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/_mapCacheSet.js

@@ -0,0 +1,22 @@
+var getMapData = require('./_getMapData');
+
+/**
+ * Sets the map `key` to `value`.
+ *
+ * @private
+ * @name set
+ * @memberOf MapCache
+ * @param {string} key The key of the value to set.
+ * @param {*} value The value to set.
+ * @returns {Object} Returns the map cache instance.
+ */
+function mapCacheSet(key, value) {
+  var data = getMapData(this, key),
+      size = data.size;
+
+  data.set(key, value);
+  this.size += data.size == size ? 0 : 1;
+  return this;
+}
+
+module.exports = mapCacheSet;