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/source-map-resolve/lib/decode-uri-component.js

@@ -0,0 +1,11 @@
+// Copyright 2017 Simon Lydell
+// X11 (“MIT”) Licensed. (See LICENSE.)
+
+var decodeUriComponent = require("decode-uri-component")
+
+function customDecodeUriComponent(string) {
+  // `decodeUriComponent` turns `+` into ` `, but that's not wanted.
+  return decodeUriComponent(string.replace(/\+/g, "%2B"))
+}
+
+module.exports = customDecodeUriComponent