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/_stackDelete.js

@@ -0,0 +1,18 @@
+/**
+ * Removes `key` and its value from the stack.
+ *
+ * @private
+ * @name delete
+ * @memberOf Stack
+ * @param {string} key The key of the value to remove.
+ * @returns {boolean} Returns `true` if the entry was removed, else `false`.
+ */
+function stackDelete(key) {
+  var data = this.__data__,
+      result = data['delete'](key);
+
+  this.size = data.size;
+  return result;
+}
+
+module.exports = stackDelete;