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/borderRightStyle.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var isValid = require('./borderStyle').isValid;
+module.exports.isValid = isValid;
+
+module.exports.definition = {
+  set: function(v) {
+    if (isValid(v)) {
+      if (v.toLowerCase() === 'none') {
+        v = '';
+        this.removeProperty('border-right-width');
+      }
+      this._setProperty('border-right-style', v);
+    }
+  },
+  get: function() {
+    return this.getPropertyValue('border-right-style');
+  },
+  enumerable: true,
+  configurable: true,
+};