Only print user tables for deployed SREs

[craddm]

:white_check_mark: Checklist

• [x] You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
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:vertical_traffic_light: Depends on

:arrow_heading_up: Summary

Checks whether if the requested SRE is in the list of deployed projects before trying to list users, and returns an informational message if not.

:closed_umbrella: Related issues

Closes #2082

:microscope: Tests

Tested locally

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/commands
users.py
Project Total

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