Improve Pulumi error messages

[craddm]

✅ Checklist

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⤴️ Summary

Fixes incorrect error messages for the deployment name argument passed to dsh init
Adds an error when the backend has not been initialised before attempting to run commands that rely on it (e.g. dsh admin list-users)

Note - this PR also updates the relevant .devcontainer to Pulumi version 3.80.0

🌂 Related issues

Closes #1623

🔬 Tests

Tested locally:

[jemrobinson]

It looks like this has been based off an older branch of python-migration. Possibly this is because you used a newer version of develop?

[craddm]

It shouldn't be - I updated my local copy of python-migration and merged it into this branch just before making the PR

Edit: ah, which makes it a mess and hard to see what I actually changed, right

[jemrobinson]

I've merged develop into the upstream python-migration and now this looks more sensible. hmm, no it doesn't. Can you git reset --hard 283f0e93e to drop your last merge commit?

[jemrobinson]

LGTM

[jemrobinson]

LGTM