Remove back link from 404 error page (#23222967)

I've kept it for 400/403/500/csrffail for now, because they also have a
"try again" link. Yes, both things have browser buttons, but they make
it a *little* clearer to technical users what one could to next, and
especially on csrffail, "step back" is always possible and possibly actually
helpful.

src/pretix/base/invoicing/pdf.py

@@ -148,10 +148,6 @@ class NumberedCanvas(Canvas):
         self.restoreState()
 
 
-class InvoiceNotReadyException(Exception):
-    pass
-
-
 class BaseInvoiceRenderer:
     """
     This is the base class for all invoice renderers.

src/pretix/base/services/invoices.py

@@ -51,7 +51,6 @@ from django_scopes import scope, scopes_disabled
 from i18nfield.strings import LazyI18nString
 
 from pretix.base.i18n import language
-from pretix.base.invoicing.pdf import InvoiceNotReadyException
 from pretix.base.invoicing.transmission import (
     get_transmission_types, transmission_providers,
 )
@@ -505,7 +504,7 @@ def generate_invoice(order: Order, trigger_pdf=True):
     return invoice
 
 
-@app.task(base=TransactionAwareTask, throws=(InvoiceNotReadyException,))
+@app.task(base=TransactionAwareTask)
 def invoice_pdf_task(invoice: int):
     with scopes_disabled():
         i = Invoice.objects.get(pk=invoice)

src/pretix/base/templates/404.html

@@ -8,9 +8,6 @@
         <h1>{% trans "Not found" %}</h1>
         <p>{% trans "I'm afraid we could not find the the resource you requested." %}</p>
         <p>{{ exception }}</p>
-        <p class="links">
-            <a id='goback' href='#'>{% trans "Take a step back" %}</a>
-        </p>
         {% if request.user.is_staff and not staff_session %}
             <form action="{% url 'control:user.sudo' %}?next={{ request.path|add:"?"|add:request.GET.urlencode|urlencode }}" method="post">
                 <p>