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/pn/test/custom.js

@@ -0,0 +1,20 @@
+// Verify that custom promises work as advertised.
+var PrPromise = require('prfun');
+var assert = require('../assert');
+var fs = require('../fs');
+
+describe('Verify that custom promises can be used', function() {
+    var oldPromise;
+    before(function() {
+        oldPromise = require('../_promise')();
+        require('../_promise')(PrPromise);
+    });
+    after(function() {
+        require('../_promise')(oldPromise);
+    });
+
+    it('exists should use the custom promise type', function() {
+        var p = fs.exists('abc');
+        assert.equal(p instanceof PrPromise, true);
+    });
+});