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/_setToArray.js

@@ -0,0 +1,18 @@
+/**
+ * Converts `set` to an array of its values.
+ *
+ * @private
+ * @param {Object} set The set to convert.
+ * @returns {Array} Returns the values.
+ */
+function setToArray(set) {
+  var index = -1,
+      result = Array(set.size);
+
+  set.forEach(function(value) {
+    result[++index] = value;
+  });
+  return result;
+}
+
+module.exports = setToArray;