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/_baseFindIndex.js

@@ -0,0 +1,24 @@
+/**
+ * The base implementation of `_.findIndex` and `_.findLastIndex` without
+ * support for iteratee shorthands.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {Function} predicate The function invoked per iteration.
+ * @param {number} fromIndex The index to search from.
+ * @param {boolean} [fromRight] Specify iterating from right to left.
+ * @returns {number} Returns the index of the matched value, else `-1`.
+ */
+function baseFindIndex(array, predicate, fromIndex, fromRight) {
+  var length = array.length,
+      index = fromIndex + (fromRight ? 1 : -1);
+
+  while ((fromRight ? index-- : ++index < length)) {
+    if (predicate(array[index], index, array)) {
+      return index;
+    }
+  }
+  return -1;
+}
+
+module.exports = baseFindIndex;