[C2] Add default error page for undefined URL in multi-URL

[ywarnier]

When using multi-URL, if the web server redirects a request to Chamilo but that URL (except localhost if no other URL is defined) is not defined in the list of managed URLs, Chamilo should redirect the request to a common error page for this.

This page (and other similar error pages) should probably be stored in assets/ somewhere (assets/css/ to be able to customize them as part of a theme, maybe?)

The page itself should be "relatively" static but load the default CSS theme selected in settings for URL=1.

At the moment, when loading a non-declared URL, we get the following error:

Call to a member function getActiveColorTheme() on null in [/var/www/chamilo/src/CoreBundle/ServiceHelper/ThemeHelper.php ](file:///var/www/chamilo/src/CoreBundle/ServiceHelper/ThemeHelper.php#L51)(line 51)

[christianbeeznest]

It is implemented in this PR #6316

Thanks for confirmation.