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/data-urls/lib/utils.js

@@ -0,0 +1,23 @@
+"use strict";
+const { percentDecode } = require("whatwg-url");
+const { atob } = require("abab");
+
+exports.stripLeadingAndTrailingASCIIWhitespace = string => {
+  return string.replace(/^[ \t\n\f\r]+/, "").replace(/[ \t\n\f\r]+$/, "");
+};
+
+exports.stringPercentDecode = input => {
+  return percentDecode(Buffer.from(input, "utf-8"));
+};
+
+exports.isomorphicDecode = input => {
+  return input.toString("binary");
+};
+
+exports.forgivingBase64Decode = data => {
+  const asString = atob(data);
+  if (asString === null) {
+    return null;
+  }
+  return Buffer.from(asString, "binary");
+};