Print a more helpful error message when token is expired

[sajith]

Currently we print a noisy backtrace (pasted below) when the token is expired. This is a little too much noise for a fairly common and expected totally expected situation.

Traceback (most recent call last):
  File "fabrictestbed-extensions/tests/integration/test_hello_fabric.py", line 19, in test_fablib_hello
    fablib = FablibManager()
             ^^^^^^^^^^^^^^^
  File "fabrictestbed-extensions/fabrictestbed_extensions/fablib/fablib.py", line 792, in __init__
    self.build_slice_manager()
  File "fabrictestbed-extensions/fabrictestbed_extensions/fablib/fablib.py", line 981, in build_slice_manager
    raise e
  File "fabrictestbed-extensions/fabrictestbed_extensions/fablib/fablib.py", line 967, in build_slice_manager
    self.slice_manager = SliceManager(
                         ^^^^^^^^^^^^^
  File "fabrictestbed-extensions/venv/lib64/python3.11/site-packages/fabrictestbed/slice_manager/slice_manager.py", line 71, in __init__
    self.initialize()
  File "fabrictestbed-extensions/venv/lib64/python3.11/site-packages/fabrictestbed/slice_manager/slice_manager.py", line 80, in initialize
    self.__load_tokens()
  File "fabrictestbed-extensions/venv/lib64/python3.11/site-packages/fabrictestbed/slice_manager/slice_manager.py", line 127, in __load_tokens
    self.refresh_tokens(refresh_token=refresh_token)
  File "fabrictestbed-extensions/venv/lib64/python3.11/site-packages/fabrictestbed/slice_manager/slice_manager.py", line 164, in refresh_tokens
    raise SliceManagerException(tokens.get(CredmgrProxy.ERROR))
fabrictestbed.slice_manager.slice_manager.SliceManagerException: b'{\n    "errors": [\n        {\n            "details": "\\"invalid_token\\"\\nerror_description=\\"token not found\\"\\n",\n            "message": "Internal Server Error"\n        }\n    ],\n    "size": 1,\n    "status": 500,\n    "type": "error"\n}'

The error message contained in SliceManagerException in this specific backtrace isn't exactly friendly, because it contains a bytes representation of JSON. That "internal server error" and format of the message suggests that Credential Manager (or/and CM proxy?) did not handle the expired token well. It would be helpful if that error message is in a more usable shape.