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/_mapToArray.js

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