Added noindex http headers

[staabm]

Its redundant a bit, but better be safe then sorry.

Proxies/webserver might override the http status code and therefore error pages could show up in e.g. google search index

https://www.google.de/search?q=Whoops%21+There+was+an+error

Refs #604

[staabm]

Not 100% sure this header should better be sent in writeToOutputNow()

whoops/src/Whoops/Run.php

Line 400 in 45dad4e

if ($this->sendHttpCode() && \Whoops\Util\Misc::canSendHeaders()) {

[kktsvetkov]

Why not add <meta name="robots" content="noindex" /> in HTML output as well ? Meta tags will be valid even after headers_sent()

[staabm]

The pretty-page-handler already contains this markup:
https://github.com/filp/whoops/blob/45dad4e41eff66217626029f4172b04dc4025442/src/Whoops/Resources/views/layout.html.php

This http-header has the benefit of also working for other handlers