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/fsevents/install.js

@@ -0,0 +1,7 @@
+if (process.platform === 'darwin') {
+  var spawn = require('child_process').spawn;
+  var args = ['install', '--fallback-to-build'];
+  var options = {stdio: 'inherit'};
+  var child = spawn(require.resolve('node-pre-gyp/bin/node-pre-gyp'), args, options);
+  child.on('close', process.exit);
+}