fix(instances): failures reach the user; no redirect to other sites

Web views:
- 34 views returned to request.META["HTTP_REFERER"] + "#tab": without
  a Referer they failed with a TypeError (500), and a Referer from
  another site was followed (open redirect). They return through
  _back(), which keeps a same-site Referer and otherwise opens the VM
  page at the tab. get_safe_redirect uses the same check. Rejected disk
  options also return to the VM's disks tab instead of the VM list.
- migrate_instance returned silently when the target was the current
  compute or unreachable, so the web view logged "Instance is migrated"
  and the API answered "instance migrate is started". It now raises;
  the view reports the error and logs only a completed migration, and
  an unknown compute id is a 404 instead of a 500. The automatic
  migration after a clone skips a target that is the current compute.
- add_owner and del_owner failed with a 500 without their ids, add_owner
  also when the user was already an owner, and del_owner deleted the
  given ownership record even when it belonged to another VM. Ids are
  validated, a record of another VM is a 404, and a repeated owner is
  reported.
- Broad handlers that hid programming errors are narrowed: clone catches
  libvirt, value and OS errors (a lock timeout included), ownership
  lookups catch DoesNotExist. The DRBD status no longer needs its bare
  except: the VM name is escaped in the regex, which was the only thing
  that could fail there.

API:
- A libvirt error under /api/ is JSON with status 400, and a lock
  timeout 409, instead of the HTML error page. This lives in the
  existing ExceptionMiddleware, so existing settings need no change.
- Volume create/delete on an inactive pool answer 409, and invalid
  volume data 400 with the serializer errors, instead of 200.

The new tests cover every view that returns with _back() without a
Referer and with a foreign one, the migrate failures on web and API,
the owner ids, the API error format and the volume statuses; nine of
the ten fail on the previous code.

webvirtcloud/middleware.py

@@ -1,8 +1,11 @@
 from django.contrib import messages
+from django.http import JsonResponse
 from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from libvirt import libvirtError
 
+API_PREFIX = "/api/"
+
 
 class ExceptionMiddleware:
     def __init__(self, get_response):
@@ -12,6 +15,13 @@ class ExceptionMiddleware:
         return self.get_response(request)
 
     def process_exception(self, request, exception):
+        # API clients get JSON with a failure status, not the HTML error page.
+        if request.path_info.startswith(API_PREFIX):
+            if isinstance(exception, libvirtError):
+                return JsonResponse({"detail": str(exception)}, status=400)
+            if isinstance(exception, TimeoutError):
+                return JsonResponse({"detail": "Operation is busy. Please retry."}, status=409)
+            return None
         if isinstance(exception, libvirtError):
             messages.error(
                 request,