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/cssstyle/lib/properties/paddingLeft.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var padding = require('./padding.js');
+var parsers = require('../parsers.js');
+
+module.exports.definition = {
+  set: parsers.subImplicitSetter('padding', 'left', padding.isValid, padding.parser),
+  get: function() {
+    return this.getPropertyValue('padding-left');
+  },
+  enumerable: true,
+  configurable: true,
+};