response: Replace json_unauthorized with UnauthorizedError.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/lib/exceptions.py

@@ -36,6 +36,7 @@ class ErrorCode(Enum):
     PASSWORD_AUTH_DISABLED = auto()
     PASSWORD_RESET_REQUIRED = auto()
     AUTHENTICATION_FAILED = auto()
+    UNAUTHORIZED = auto()
 
 
 class JsonableError(Exception):
@@ -124,6 +125,28 @@ class JsonableError(Exception):
         return self.msg
 
 
+class UnauthorizedError(JsonableError):
+    code: ErrorCode = ErrorCode.UNAUTHORIZED
+    http_status_code: int = 401
+
+    def __init__(self, msg: Optional[str] = None, www_authenticate: Optional[str] = None) -> None:
+        if msg is None:
+            msg = _("Not logged in: API authentication or user session required")
+        super().__init__(msg)
+        if www_authenticate is None:
+            self.www_authenticate = 'Basic realm="zulip"'
+        elif www_authenticate == "session":
+            self.www_authenticate = 'Session realm="zulip"'
+        else:
+            raise AssertionError("Invalid www_authenticate value!")
+
+    @property
+    def extra_headers(self) -> Dict[str, Any]:
+        extra_headers_dict = super().extra_headers
+        extra_headers_dict["WWW-Authenticate"] = self.www_authenticate
+        return extra_headers_dict
+
+
 class StreamDoesNotExistError(JsonableError):
     code = ErrorCode.STREAM_DOES_NOT_EXIST
     data_fields = ["stream"]