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/_stackClear.js

@@ -0,0 +1,15 @@
+var ListCache = require('./_ListCache');
+
+/**
+ * Removes all key-value entries from the stack.
+ *
+ * @private
+ * @name clear
+ * @memberOf Stack
+ */
+function stackClear() {
+  this.__data__ = new ListCache;
+  this.size = 0;
+}
+
+module.exports = stackClear;