[Bug] MissingConfigException cannot be correctly deserialized, due to lack of missing_cfg_file ctor default

[gazonk]

🐛 Bug

Description

in https://github.com/facebookresearch/hydra/blob/main/hydra/errors.py
the missing_cfg_file parameter of the MissingConfigException should be defaulted to None since it is optional, otherwise deserialization will fail.

Checklist

• I checked on latest commit [7bc2b1a] of errors.py (7bc2b1a)
• I created a minimal repro (See this for tips).

To reproduce

** Minimal Code/Config snippet to reproduce **

import pickle
import hydra
e = hydra.errors.MissingConfigException("missing", "file")
x = pickle.dumps(e)
y = pickle.loads(x)

** Stack trace/error message **

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'missing_cfg_file'

Expected Behavior

successful deserialization:

>>> y
MissingConfigException('missing')

System information

• Hydra Version : hydra-core==1.3.1
• Python version : Python 3.8.13
• Virtual environment type and version : None
• Operating system : Ubuntu 22.04.1 LT

Additional context

This exception was serialized/deserialized when using ray tune.

[Jasha10]

Thanks for the bug report, @gazonk! I can reproduce the issue on my local machine.