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/borderLeft.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var shorthandSetter = require('../parsers').shorthandSetter;
+var shorthandGetter = require('../parsers').shorthandGetter;
+
+var shorthand_for = {
+  'border-left-width': require('./borderLeftWidth'),
+  'border-left-style': require('./borderLeftStyle'),
+  'border-left-color': require('./borderLeftColor'),
+};
+
+module.exports.definition = {
+  set: shorthandSetter('border-left', shorthand_for),
+  get: shorthandGetter('border-left', shorthand_for),
+  enumerable: true,
+  configurable: true,
+};