Coveralls: Add back errors.js to sourcefiles (#1664)

Looks like I accidentally reverted my fix while rebasing
d6cd0134

scripts/coverage.js

@@ -8,7 +8,7 @@ var Mocha = require('mocha'),
   mkdirp = require('mkdirp'),
   coveralls = require('coveralls'),
   istanbul = require('istanbul'),
-  sourcefiles = ['index.js', 'extensions.js', 'render.js'],
+  sourcefiles = ['index.js', 'extensions.js', 'render.js', 'errors.js'],
   summary= istanbul.Report.create('text-summary'),
   lcov = istanbul.Report.create('lcovonly', { dir: path.join('coverage') }),
   html = istanbul.Report.create('html', { dir: path.join('coverage', 'html') });