[Fix] sync: ensure that the path is a string, explicitly.

Also, make the sync/async error messages here the same, and ensure it works in node 0.6 (which doesn’t throw on non-strings in `path.join`)

Author: ljharb

lib/async.js

@@ -12,7 +12,7 @@ module.exports = function resolve(x, options, callback) {
         opts = {};
     }
     if (typeof x !== 'string') {
-        var err = new TypeError('path must be a string');
+        var err = new TypeError('Path must be a string.');
         return process.nextTick(function () {
             cb(err);
         });

lib/sync.js

@@ -5,6 +5,9 @@ var caller = require('./caller.js');
 var nodeModulesPaths = require('./node-modules-paths.js');
 
 module.exports = function (x, options) {
+    if (typeof x !== 'string') {
+        throw new TypeError('Path must be a string.');
+    }
     var opts = options || {};
     var isFile = opts.isFile || function (file) {
         try {