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/_baseSortedUniq.js

@@ -0,0 +1,30 @@
+var eq = require('./eq');
+
+/**
+ * The base implementation of `_.sortedUniq` and `_.sortedUniqBy` without
+ * support for iteratee shorthands.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {Function} [iteratee] The iteratee invoked per element.
+ * @returns {Array} Returns the new duplicate free array.
+ */
+function baseSortedUniq(array, iteratee) {
+  var index = -1,
+      length = array.length,
+      resIndex = 0,
+      result = [];
+
+  while (++index < length) {
+    var value = array[index],
+        computed = iteratee ? iteratee(value) : value;
+
+    if (!index || !eq(computed, seen)) {
+      var seen = computed;
+      result[resIndex++] = value === 0 ? 0 : value;
+    }
+  }
+  return result;
+}
+
+module.exports = baseSortedUniq;