Return 400 for a syntactically invalid pos after all

Removing M_UNKNOWN_POS is about well-formed positions the server no
longer recognises; a malformed token is a plain client error like
everywhere else. Reverts part of 20feb1ef15 and keeps a test pinning
the 400.

synapse/rest/client/paginated_sync.py

@@ -23,7 +23,6 @@ sync servlet; only the request parsing and the top-level response differ.
 import logging
 from typing import TYPE_CHECKING
 
-from synapse.api.errors import SynapseError
 from synapse.http.server import HttpServer
 from synapse.http.servlet import (
     parse_and_validate_json_object_from_request,
@@ -81,18 +80,12 @@ class MSC4525PaginatedSyncRestServlet(SlidingSyncRestServlet):
 
         from_token = None
         if from_token_string is not None:
-            try:
-                from_token = await SlidingSyncStreamToken.from_string(
-                    self.store, from_token_string
-                )
-            except SynapseError:
-                # There is no client error path in this API: an unparsable
-                # `pos` is treated the same as an unrecognised one - as
-                # absent, so the connection starts afresh.
-                logger.info(
-                    "Unparsable paginated sync pos for %s; starting the connection afresh",
-                    user,
-                )
+            # A syntactically invalid pos is a plain 400, as elsewhere. Only a
+            # well-formed pos the server doesn't recognise restarts the
+            # connection (see the handler).
+            from_token = await SlidingSyncStreamToken.from_string(
+                self.store, from_token_string
+            )
 
         body = parse_and_validate_json_object_from_request(
             request, MSC4525PaginatedSyncBody

tests/rest/client/sliding_sync/test_msc4525_paginated_sync.py

@@ -339,9 +339,17 @@ class MSC4525PaginatedSyncTestCase(unittest.HomeserverTestCase):
         for room_id, room_response in response["rooms"].items():
             self.assertTrue(room_response.get("initial"), room_id)
 
-        # An entirely unparsable pos is treated the same way, not a 400.
-        response = self._sync(body, pos="not a token at all")
-        self.assertEqual(set(response["rooms"].keys()), set(room_ids))
+        # A syntactically invalid pos, by contrast, is a plain 400: only
+        # well-formed-but-unrecognised positions restart the connection.
+        path = (
+            self.sync_endpoint
+            + "?"
+            + urllib.parse.urlencode({"timeout": "0", "pos": "not a token at all"})
+        )
+        channel = self.make_request(
+            method="POST", path=path, content=body, access_token=self.tok
+        )
+        self.assertEqual(channel.code, 400, channel.json_body)
 
     def test_cold_start_backlog_not_starved_by_live_traffic(self) -> None:
         """Rooms never sent on the connection get a reserved slice of every