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/mkdirp/node_modules/minimist/test/dotted.js

@@ -0,0 +1,16 @@
+var parse = require('../');
+var test = require('tape');
+
+test('dotted alias', function (t) {
+    var argv = parse(['--a.b', '22'], {default: {'a.b': 11}, alias: {'a.b': 'aa.bb'}});
+    t.equal(argv.a.b, 22);
+    t.equal(argv.aa.bb, 22);
+    t.end();
+});
+
+test('dotted default', function (t) {
+    var argv = parse('', {default: {'a.b': 11}, alias: {'a.b': 'aa.bb'}});
+    t.equal(argv.a.b, 11);
+    t.equal(argv.aa.bb, 11);
+    t.end();
+});