feat(auth): send a request again when its token lapsed in the queue

A request can wait to be served for longer than the access token it was
sent with lives, and comes back refused for a token that was valid when
it left. Uploads that take minutes make that wait ordinary.

- tell an expired token apart from any other refusal by the code the
  API returns, and retry only that one, once
- prefer a token another process left behind over minting a second: it
  saves a round trip, and a rotating provider would retire one that is
  still in use. Assign it rather than read it, since the setter is what
  rewrites the session header the retry will carry
- settle the token before the request so the one compared afterwards is
  the one that was actually sent
- report an overloaded server when the second attempt is refused too,
  and let API errors out of the upload path with their own type

mdrsclient/exceptions.py

@@ -28,6 +28,18 @@ class UnauthorizedException(MDRSException):
     pass
 
 
+class TokenExpiredException(UnauthorizedException):
+    """Thrown when the access token was still valid when sent but had expired on arrival"""
+
+    pass
+
+
+class ServerBusyException(MDRSException):
+    """Thrown when the server did not start handling a request before its token lapsed"""
+
+    pass
+
+
 class ForbiddenException(MDRSException):
     """Thrown when the current user does not have enough privileges to access the resource"""