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/width.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var parseMeasurement = require('../parsers').parseMeasurement;
+
+function parse(v) {
+  if (String(v).toLowerCase() === 'auto') {
+    return 'auto';
+  }
+  if (String(v).toLowerCase() === 'inherit') {
+    return 'inherit';
+  }
+  return parseMeasurement(v);
+}
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('width', parse(v));
+  },
+  get: function() {
+    return this.getPropertyValue('width');
+  },
+  enumerable: true,
+  configurable: true,
+};