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/_SetCache.js

@@ -0,0 +1,27 @@
+var MapCache = require('./_MapCache'),
+    setCacheAdd = require('./_setCacheAdd'),
+    setCacheHas = require('./_setCacheHas');
+
+/**
+ *
+ * Creates an array cache object to store unique values.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [values] The values to cache.
+ */
+function SetCache(values) {
+  var index = -1,
+      length = values == null ? 0 : values.length;
+
+  this.__data__ = new MapCache;
+  while (++index < length) {
+    this.add(values[index]);
+  }
+}
+
+// Add methods to `SetCache`.
+SetCache.prototype.add = SetCache.prototype.push = setCacheAdd;
+SetCache.prototype.has = setCacheHas;
+
+module.exports = SetCache;