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/util-deprecate/History.md

@@ -0,0 +1,16 @@
+
+1.0.2 / 2015-10-07
+==================
+
+  * use try/catch when checking `localStorage` (#3, @kumavis)
+
+1.0.1 / 2014-11-25
+==================
+
+  * browser: use `console.warn()` for deprecation calls
+  * browser: more jsdocs
+
+1.0.0 / 2014-04-30
+==================
+
+  * initial commit