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/resolve-url.js

@@ -0,0 +1,12 @@
+// Copyright 2014 Simon Lydell
+// X11 (“MIT”) Licensed. (See LICENSE.)
+
+var url = require("url")
+
+function resolveUrl(/* ...urls */) {
+  return Array.prototype.reduce.call(arguments, function(resolved, nextUrl) {
+    return url.resolve(resolved, nextUrl)
+  })
+}
+
+module.exports = resolveUrl