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/borderStyle.js

@@ -0,0 +1,38 @@
+'use strict';
+
+var implicitSetter = require('../parsers').implicitSetter;
+
+// the valid border-styles:
+var styles = [
+  'none',
+  'hidden',
+  'dotted',
+  'dashed',
+  'solid',
+  'double',
+  'groove',
+  'ridge',
+  'inset',
+  'outset',
+];
+
+module.exports.isValid = function parse(v) {
+  return typeof v === 'string' && (v === '' || styles.indexOf(v) !== -1);
+};
+var isValid = module.exports.isValid;
+
+var parser = function(v) {
+  if (isValid(v)) {
+    return v.toLowerCase();
+  }
+  return undefined;
+};
+
+module.exports.definition = {
+  set: implicitSetter('border', 'style', isValid, parser),
+  get: function() {
+    return this.getPropertyValue('border-style');
+  },
+  enumerable: true,
+  configurable: true,
+};