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/_countHolders.js

@@ -0,0 +1,21 @@
+/**
+ * Gets the number of `placeholder` occurrences in `array`.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {*} placeholder The placeholder to search for.
+ * @returns {number} Returns the placeholder count.
+ */
+function countHolders(array, placeholder) {
+  var length = array.length,
+      result = 0;
+
+  while (length--) {
+    if (array[length] === placeholder) {
+      ++result;
+    }
+  }
+  return result;
+}
+
+module.exports = countHolders;