Add RpcException.StatusCode property

[jskeet]

This is purely for convenience - using e.Status.StatusCode looks
clunky compared with e.StatusCode, and this is the property most
likely to be used for exception filtering etc.

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[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

• User @jskeet isn't covered by a CLA. They will need to complete the form at https://identity.linuxfoundation.org/projects/cncf

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[jskeet]

(CLA now signed.)

[jtattermusch]

LGTM.

[jskeet]

@jtattermusch: Do I need to worry about the interop CI failures?

[mehrdada]

@jskeet I checked the interop failures. No need to worry about them now. It's a known failure.

[jskeet]

Looks like I can't merge anyway. It's been a while since I've sent a PR for a repo I don't have write access to :)