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/minimist/test/stop_early.js

@@ -0,0 +1,15 @@
+var parse = require('../');
+var test = require('tape');
+
+test('stops parsing on the first non-option when stopEarly is set', function (t) {
+    var argv = parse(['--aaa', 'bbb', 'ccc', '--ddd'], {
+        stopEarly: true
+    });
+
+    t.deepEqual(argv, {
+        aaa: 'bbb',
+        _: ['ccc', '--ddd']
+    });
+
+    t.end();
+});