npm and error messages

node_modules/clean-css/lib/optimizer/level-2/invalid-property-error.js

@@ -0,0 +1,10 @@
+function InvalidPropertyError(message) {
+  this.name = 'InvalidPropertyError';
+  this.message = message;
+  this.stack = (new Error()).stack;
+}
+
+InvalidPropertyError.prototype = Object.create(Error.prototype);
+InvalidPropertyError.prototype.constructor = InvalidPropertyError;
+
+module.exports = InvalidPropertyError;