[Bugfix] Preserve original ImportError in gRPC server entrypoint

[CatherineSue]

Purpose

Fix misleading error message when smg-grpc-servicer is installed but has a broken internal import (e.g. version mismatch between servicer and vllm, or a missing transitive dependency like grpcio).

Currently from None suppresses the original traceback, so the user only sees:

ImportError: smg-grpc-servicer is required for gRPC mode.
Install it with: pip install vllm[grpc]

This is misleading when the package IS installed but an internal import fails — the real error is hidden.

Test Plan

Manual verification: introduce a broken import inside smg_grpc_servicer and confirm the chained traceback is now visible.

Test Result

After this change, the original ImportError is chained and visible:

ImportError: <real error details>

The above exception was the direct cause of the following exception:

ImportError: gRPC mode requires smg-grpc-servicer.
If not installed, run: pip install vllm[grpc].
If already installed, there may be a broken import due to a
version mismatch — see the chained exception above for details.

---

• The purpose of the PR: preserve original traceback for gRPC import errors
• The test plan: manual verification of error output
• The test results: chained exception now visible

[gemini-code-assist]

Code Review

This pull request improves the error handling for gRPC-related imports in vllm/entrypoints/grpc_server.py. The updated logic now captures the original ImportError and provides a more descriptive error message to help users diagnose potential version mismatches or installation issues. I have no feedback to provide as there are no review comments.