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/yargs/lib/middleware.js

@@ -0,0 +1,10 @@
+module.exports = function (globalMiddleware, context) {
+  return function (callback) {
+    if (Array.isArray(callback)) {
+      Array.prototype.push.apply(globalMiddleware, callback)
+    } else if (typeof callback === 'function') {
+      globalMiddleware.push(callback)
+    }
+    return context
+  }
+}