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/tmpl/readme.md

@@ -0,0 +1,10 @@
+tmpl [![Build Status](https://secure.travis-ci.org/nshah/nodejs-tmpl.png)](http://travis-ci.org/nshah/nodejs-tmpl)
+====
+
+Simple string formatting using `{}`.
+
+```javascript
+assert.equal(
+  tmpl('the answer is {answer}', { answer: 42 }),
+  'the answer is 42')
+```