[ml] Revert Exception to MlException change in activity.py

[diondrapeck]

Description

In #35342, replacing Exception with MlException to appease the next-pylint restriction on using the BaseException class caused unittest failures for azureml-featurestore due to differences in the message attributes for the two classes. In this PR, I am reverting to the use of Exception and simply silencing the pylint error on that line.

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[azure-sdk]

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azure-ai-ml