Chore: Add missing 'error.js' to coverage list

scripts/coverage.js

@@ -10,7 +10,7 @@ var Mocha = require('mocha'),
     Instrumenter = require('istanbul').Instrumenter,
     Report = require('istanbul').Report,
     Collector = new require('istanbul').Collector,
-    sourcefiles = ['index.js', 'extensions.js', 'render.js'],
+    sourcefiles = ['index.js', 'extensions.js', 'render.js', 'errors.js'],
     summary= Report.create('text-summary'),
     lcov = Report.create('lcovonly', { dir: path.join('coverage') }),
     html = Report.create('html', { dir: path.join('coverage', 'html') });