imaplib: downgrade idle-denied exception to error

This makes it easier for client code to distinguish a temporary
rejection of the IDLE command from a server responding incorrectly to
IDLE.

Lib/imaplib.py

@@ -1413,6 +1413,9 @@ def __enter__(self):
             # this occurs.
             while resp := imap._get_response():
                 if imap.tagged_commands[self._tag]:
+                    typ, data = imap.tagged_commands.pop(self._tag)
+                    if typ == 'NO':
+                        raise imap.error(f'idle denied: {data}')
                     raise imap.abort(f'unexpected status response: {resp}')
 
             if __debug__ and imap.debug >= 4:

Lib/test/test_imaplib.py

@@ -208,6 +208,12 @@ def cmd_UNSELECT(self, tag, args):
             self._send_tagged(tag, 'BAD', 'No mailbox selected')
 
 
+class IdleCmdDenyHandler(SimpleIMAPHandler):
+    capabilities = 'IDLE'
+    def cmd_IDLE(self, tag, args):
+        self._send_tagged(tag, 'NO', 'IDLE is not allowed at this time')
+
+
 class IdleCmdHandler(SimpleIMAPHandler):
     capabilities = 'IDLE'
     def cmd_IDLE(self, tag, args):
@@ -529,6 +535,13 @@ def test_idle_capability(self):
             with client.idle():
                 pass
 
+    def test_idle_denied(self):
+        client, _ = self._setup(IdleCmdDenyHandler)
+        client.login('user', 'pass')
+        with self.assertRaises(imaplib.IMAP4.error):
+            with client.idle() as idler:
+                pass
+
     def test_idle_iter(self):
         client, _ = self._setup(IdleCmdHandler)
         client.login('user', 'pass')