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/readable-stream/writable.js

@@ -0,0 +1,8 @@
+var Stream = require("stream")
+var Writable = require("./lib/_stream_writable.js")
+
+if (process.env.READABLE_STREAM === 'disable') {
+  module.exports = Stream && Stream.Writable || Writable
+} else {
+  module.exports = Writable
+}