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/performance-now/src/index.d.ts

@@ -0,0 +1,8 @@
+// This file describes the package to typescript.
+
+/**
+ * Returns the number of milliseconds since the page was loaded (if browser)
+ * or the node process was started.
+ */
+declare function now(): number;
+export = now;