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/nonstring.js

@@ -0,0 +1,9 @@
+var test = require('tape');
+var resolve = require('../');
+
+test('nonstring', function (t) {
+    t.plan(1);
+    resolve(555, function (err, res, pkg) {
+        t.ok(err);
+    });
+});