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/resolve/test/subdirs.js

@@ -0,0 +1,13 @@
+var test = require('tape');
+var resolve = require('../');
+var path = require('path');
+
+test('subdirs', function (t) {
+    t.plan(2);
+
+    var dir = path.join(__dirname, '/subdirs');
+    resolve('a/b/c/x.json', { basedir: dir }, function (err, res) {
+        t.ifError(err);
+        t.equal(res, path.join(dir, 'node_modules/a/b/c/x.json'));
+    });
+});