feat(vrtManager): the libvirt layer's messages show in the reader's language

vrtManager's errors (some 140: a disk that does not move, a network
address outside its subnet, a host device given to another VM, the
disks a migration cannot copy) were English f-strings, so no catalog
could translate them, on pages or in jobs.

They are marked texts with their values now; a variant that differs by
a word (internal or external, running or persistent, with a subnet or
without) is a whole message of its own, and a list of problems is a
list of messages. OperationError reads in the active language, so the
pages that show one translate it as they are; its args stay English, and
so do the logs and the audit log. Host device problems and the Migrate
tab's reasons a disk is not copied are messages too.

vrtManager/util.py

@@ -1,3 +1,4 @@
+from django.utils.translation import gettext_noop
 import ipaddress
 import random
 import secrets
@@ -8,23 +9,47 @@ import libvirt
 import lxml.etree as etree
 
 from django.conf import settings
+from django.utils.translation import gettext
 
 
 
+def message(text, **values):
+    """Our text, marked with gettext_noop, with %(name)s for each of values.
+    A value may be a list of messages: they are put in its place one after
+    another."""
+    return {"text": str(text), "values": values}
+
+
+def render(text, values, translate):
+    """text with its values, through translate (gettext, or str for English)."""
+    def shown(value):
+        if isinstance(value, list):
+            return "; ".join(render(m["text"], m["values"], translate) for m in value)
+        if isinstance(value, OperationError):
+            return render(value.text, value.values, translate)
+        return value
+    text = translate(text)
+    return text % {key: shown(value) for key, value in values.items()} if values else text
+
+
 class OperationError(libvirt.libvirtError):
-    """An error with WebVirtCloud's own message: text, with %(name)s for each
-    of values. str() is the English message; a page or a job shows text
-    translated, with the values (operations.utils.said).
+    """An error with WebVirtCloud's own message (see message()). Its args are
+    the English message, for logs and the audit log; str() is the message in
+    the active language, English outside a request. A job keeps the text and
+    values (operations.utils.said) and shows them in its reader's language.
 
     libvirtError(msg) reports the thread's last libvirt error instead of msg
     when there is one, so an earlier, unrelated failure would be shown.
     """
 
     def __init__(self, text, **values):
-        Exception.__init__(self, text % values if values else text)
         self.text, self.values = str(text), values
+        Exception.__init__(self, render(self.text, values, str))
         self.err = None
 
+    def __str__(self):
+        return render(self.text, self.values, gettext)
+
 
 class ConnectionFailed(OperationError):
     """The host could not be connected to; the message may name it and how it
@@ -112,9 +137,9 @@ def parse_xml(xml):
     try:
         tree = etree.fromstring(xml.encode(), parser)  # bytes: the XML may declare its encoding
     except etree.XMLSyntaxError as err:
-        raise OperationError(f"The XML is not valid: {err}") from None
+        raise OperationError(gettext_noop("The XML is not valid: %(err)s"), err=err) from None
     if tree.getroottree().docinfo.internalDTD is not None:
-        raise OperationError("The XML may not declare a DTD")
+        raise OperationError(gettext_noop("The XML may not declare a DTD"))
     return tree