feat: add shared exception hierarchy for Kubeflow SDK

[Goku2099]

Introduce a shared exception module (kubeflow.common.exceptions) with a base KubeflowError and a minimal set of specific exceptions.
This lays the foundation for consistent and precise error handling across SDK clients. Migration of existing clients will be done in follow-up PRs.

#458

[google-oss-prow]

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[github-actions]

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[Copilot]

Pull request overview

This PR introduces a shared exception hierarchy under kubeflow.common to standardize error types across Kubeflow SDK clients, aligning with Issue #458’s goal of more precise, catchable domain errors.

Changes:

• Added kubeflow.common.exceptions defining KubeflowError plus a minimal set of domain-specific exceptions.
• Added kubeflow.common.__init__ exports so exceptions can be imported from kubeflow.common.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
kubeflow/common/exceptions.py	Adds the shared exception class hierarchy (base + initial set).
kubeflow/common/__init__.py	Re-exports the exception types as part of the kubeflow.common public API.

[Goku2099]

cc @tariq-hasan @Shekharrajak

I have made some updates to the PR based on the initial feedback and kept the scope focused on introducing the shared exception hierarchy.

Would appreciate your thoughts on the approach whenever you get a chance.