GH-22: Using starlette like exception handling

That is:
- raising starlette.authentication.AuthenticationError
- providing an on_error callback turning starlet 400 into 401
  to keep same api
- letting the user provide their own on_error when instantiating
  the middleware.

Author: vokimon

src/fastapi_oauth2/middleware.py

@@ -16,10 +16,13 @@
 from jose.jwt import encode as jwt_encode
 from starlette.authentication import AuthCredentials
 from starlette.authentication import AuthenticationBackend
+from starlette.authentication import AuthenticationError
 from starlette.authentication import BaseUser
 from starlette.middleware.authentication import AuthenticationMiddleware
 from starlette.requests import Request
+from starlette.requests import HTTPConnection
 from starlette.responses import PlainTextResponse
+from starlette.responses import Response
 from starlette.types import ASGIApp
 from starlette.types import Receive
 from starlette.types import Scope
@@ -111,9 +114,9 @@ async def authenticate(self, request: Request) -> Optional[Tuple[Auth, User]]:
         try:
             token_data = Auth.jwt_decode(param)
         except JOSEError as e:
-            raise OAuth2AuthenticationError(401, str(e))
+            raise AuthenticationError(str(e))
         if token_data["exp"] and token_data["exp"] < int(datetime.now(timezone.utc).timestamp()):
-            raise OAuth2AuthenticationError(401, "Token expired")
+            raise AuthenticationError("Token expired")
 
         user = User(token_data)
         auth = Auth(user.pop("scope", []))
@@ -138,6 +141,7 @@ def __init__(
             app: ASGIApp,
             config: Union[OAuth2Config, dict],
             callback: Callable[[Auth, User], Union[Awaitable[None], None]] = None,
+            on_error: Callable[[HTTPConnection, AuthenticationError], Response] | None = None,
             **kwargs,  # AuthenticationMiddleware kwargs
     ) -> None:
         """Initiates the middleware with the given configuration.
@@ -151,9 +155,13 @@ def __init__(
         elif not isinstance(config, OAuth2Config):
             raise TypeError("config is not a valid type")
         self.default_application_middleware = app
-        self.auth_middleware = AuthenticationMiddleware(app, backend=OAuth2Backend(config, callback), **kwargs)
+        self.auth_middleware = AuthenticationMiddleware(app, backend=OAuth2Backend(config, callback), on_error = on_error or self.on_error, **kwargs)
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if scope["type"] == "http":
             return await self.auth_middleware(scope, receive, send)
         await self.default_application_middleware(scope, receive, send)
+
+    @staticmethod
+    def on_error(conn: HTTPConnection, exc: Exception) -> Response:
+        return PlainTextResponse(str(exc), status_code=401)