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/test/common.js

@@ -0,0 +1,27 @@
+// Copyright 2014 Simon Lydell
+// X11 (“MIT”) Licensed. (See LICENSE.)
+
+function u(url) {
+  return "code\n/*# sourceMappingURL=" + url + " */"
+}
+
+function read(x) {
+  return function() {
+    return x
+  }
+}
+
+function Throws(x) {
+  throw new Error(x)
+}
+
+function identity(x) {
+  return x
+}
+
+module.exports = {
+  u:        u,
+  read:     read,
+  Throws:   Throws,
+  identity: identity
+}