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/test/opts_fs_sync.js

@@ -0,0 +1,27 @@
+var mkdirp = require('../');
+var path = require('path');
+var test = require('tap').test;
+var mockfs = require('mock-fs');
+var _0777 = parseInt('0777', 8);
+var _0755 = parseInt('0755', 8);
+
+test('opts.fs sync', function (t) {
+    t.plan(4);
+    
+    var x = Math.floor(Math.random() * Math.pow(16,4)).toString(16);
+    var y = Math.floor(Math.random() * Math.pow(16,4)).toString(16);
+    var z = Math.floor(Math.random() * Math.pow(16,4)).toString(16);
+    
+    var file = '/beep/boop/' + [x,y,z].join('/');
+    var xfs = mockfs.fs();
+    
+    mkdirp.sync(file, { fs: xfs, mode: _0755 });
+    xfs.exists(file, function (ex) {
+        t.ok(ex, 'created file');
+        xfs.stat(file, function (err, stat) {
+            t.ifError(err);
+            t.equal(stat.mode & _0777, _0755);
+            t.ok(stat.isDirectory(), 'target not a directory');
+        });
+    });
+});