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/lib/tmpl.js

@@ -0,0 +1,17 @@
+var INTERPOLATE = /{([\s\S]+?)}/g
+
+module.exports = function(str, data) {
+  var tmpl = 'var __p=[],print=function(){__p.push.apply(__p,arguments);};' +
+    'with(obj||{}){__p.push(\'' +
+    str.replace(/\\/g, '\\\\')
+       .replace(/'/g, "\\'")
+       .replace(INTERPOLATE, function(match, code) {
+         return "'," + code.replace(/\\'/g, "'") + ",'"
+       })
+       .replace(/\r/g, '\\r')
+       .replace(/\n/g, '\\n')
+       .replace(/\t/g, '\\t')
+       + "');}return __p.join('');"
+  var func = new Function('obj', tmpl)
+  return data ? func(data) : func
+}