fix(interfaces): an ethernet interface needs no bridge member, and unreadable host devices are reported

The form required a bridge member for an ethernet interface, which does
not use one. When the host's network devices could not be read, the error
was swallowed and every creation failed with "the device is not on this
host"; the page now shows the real error and does not try.

interfaces/views.py

@@ -1,8 +1,8 @@
-import contextlib
 from django.contrib import messages
 from django.shortcuts import render, get_object_or_404
 from django.http import HttpResponseRedirect
 from django.urls import reverse
+from django.utils.translation import gettext_lazy as _
 from libvirt import libvirtError
 from computes.models import Compute
 from interfaces.forms import AddInterface
@@ -30,9 +30,12 @@ def interfaces(request, compute_id):
         )
         ifaces = conn.get_ifaces()
 
-        netdevs = []
-        with contextlib.suppress(Exception):
+        netdevs, netdev_error = [], None
+        try:
             netdevs = conn.get_net_devices()
+        except Exception as err:
+            netdev_error = err
+            messages.error(request, _("The network devices of this host could not be read: %(error)s") % {"error": err})
 
         for iface in ifaces:
             interf = wvmInterface(
@@ -45,7 +48,8 @@ def interfaces(request, compute_id):
             ifaces_all.append(interf.get_details())
 
         if request.method == "POST":
-            if "create" in request.POST:
+            # without the host's devices nothing can be checked, so nothing is created
+            if "create" in request.POST and netdev_error is None:
                 form = AddInterface(request.POST, netdevs=netdevs)
                 if form.is_valid():
                     data = form.cleaned_data