Traceback from validation error when uploading an SRE config

:white_check_mark: Checklist

[x] I have searched open and closed issues for duplicates.
[x] This is a problem observed when deploying a Data Safe Haven.
[x] I can reproduce this with the latest version.
[x] I have read through the documentation.
[x] This isn't an open-ended question (open a discussion if it is).

:computer: System information

Operating System: Debian bookworm
Data Safe Haven version: develop fecd0ab42a8cc97f6cedb326271c11de3195e9c1

:package: Packages

List of packages

```none Paste list of packages here ```

:no_entry_sign: Describe the problem

Attempting to upload an invalid configuration file for an SRE results in a traceback rather than a clean exit. Should exit cleanly with a clear error message.

:deciduous_tree: Log messages

Relevant log messages

```none ![image](https://github.com/user-attachments/assets/b76538a1-57bd-4fa8-9cc5-f8cca40e4beb) ```

:recycle: To reproduce

Attempt to upload an invalid SRE config

alan-turing-institute / data-safe-haven

Traceback from validation error when uploading an SRE config #2210