Fixes #17864 - Show smart proxy errors on unattended fail

[dLobatog]

If your proxy is has some error (like misconfigured sudoers, puppet not
available, etc...) and you try to boot a host through unattended
mode [1], the exception will not show up when you try to fetch the
kickstart template. Instead, it will show something like No HTTP_REFERER
was set in the request to this action, so redirect_to :back could not be
called successfully.

The application controller should be aware of this situation and don't
try to redirect to back in it, instead just render the error.

[1] an example of such error - ProxyAPI::ProxyException: ERF12-0104
[ProxyAPI::ProxyException]: Unable to set PuppetCA autosign for
tonia-rippeon.lobatolan.home ([RestClient::NotAcceptable]: 406 Not
Acceptable) for proxy
https://centos7-devel.lobatolan.home:9090/puppet/ca

[mention-bot]

@dLobatog, thanks for your PR! By analyzing the history of the files in this pull request, we identified @domcleal, @ohadlevy and @ares to be potential reviewers.

[dLobatog]

[test upgrade]

[tbrisker]

Thanks @dLobatog !