Forbid locked users from using POST /login (#18594)

Discussed in the [Synapse Dev
room](https://matrix.to/#/!vcyiEtMVHIhWXcJAfl:sw1v.org/$K4UojQtvaSpxSe35TWFXtKWGoAuHwHFcKo8qn2lwxSs?via=matrix.org&via=element.io&via=envs.net)

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synapse/api/auth/internal.py

@@ -29,6 +29,7 @@ from synapse.api.errors import (
     InvalidClientTokenError,
     MissingClientTokenError,
     UnrecognizedRequestError,
+    UserLockedError,
 )
 from synapse.http.site import SynapseRequest
 from synapse.logging.opentracing import active_span, force_tracing, start_active_span
@@ -162,12 +163,7 @@ class InternalAuth(BaseAuth):
                 if not allow_locked and await self.store.get_user_locked_status(
                     requester.user.to_string()
                 ):
-                    raise AuthError(
-                        401,
-                        "User account has been locked",
-                        errcode=Codes.USER_LOCKED,
-                        additional_fields={"soft_logout": True},
-                    )
+                    raise UserLockedError()
 
                 # Deny the request if the user account has expired.
                 # This check is only done for regular users, not appservice ones.

synapse/api/errors.py

@@ -306,6 +306,20 @@ class UserDeactivatedError(SynapseError):
         )
 
 
+class UserLockedError(SynapseError):
+    """The error returned to the client when the user attempted to access an
+    authenticated endpoint, but the account has been locked.
+    """
+
+    def __init__(self) -> None:
+        super().__init__(
+            code=HTTPStatus.UNAUTHORIZED,
+            msg="User account has been locked",
+            errcode=Codes.USER_LOCKED,
+            additional_fields={"soft_logout": True},
+        )
+
+
 class FederationDeniedError(SynapseError):
     """An error raised when the server tries to federate with a server which
     is not on its federation whitelist.