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/asn1/lib/ber/index.js

@@ -0,0 +1,27 @@
+// Copyright 2011 Mark Cavage <mcavage@gmail.com> All rights reserved.
+
+var errors = require('./errors');
+var types = require('./types');
+
+var Reader = require('./reader');
+var Writer = require('./writer');
+
+
+// --- Exports
+
+module.exports = {
+
+  Reader: Reader,
+
+  Writer: Writer
+
+};
+
+for (var t in types) {
+  if (types.hasOwnProperty(t))
+    module.exports[t] = types[t];
+}
+for (var e in errors) {
+  if (errors.hasOwnProperty(e))
+    module.exports[e] = errors[e];
+}