Error page templates can be found in templates/bundles/TwigBundle/Exception.
Which template is used and how errors are displayed to end users depend on the exception type.
If a NotFoundHttpException or AccessDeniedHttpException is thrown, a
BeWelcome themed page will be shown with a friendly message, using the templates
error404.html.twig and error403.html.twig, respectively.
These two templates both extend the base BeWelcome base.html.twig template, so
they are shown in the context of the full website theme.
If an unhandled application exception is thrown, we cannot assume it is possible
to display the full website theme, so we must show a much simpler page using the
template error.html.twig.
Preferably this page should not include any external assets as we cannot
guarantee the state of the external environment. Any required CSS/JS should be
inline or in the <head> tag, and images embedded using the
data URI scheme.
In a dev environment or with debug mode active Symfony error pages are shown instead of customized BeWelcome pages.