Fix terminal state on exit when Prepare fails.

[dbishop]

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Description

Clean up terminal state when Prepare fails before exiting.

Motivation and Context

I'm not sure, but this could be the same issue as #371

I noticed my terminal getting messed up when running "warp delete" on a bucket that happened to be empty, making me have to run "reset".

This patch fixed the issue for me.

How to test this PR?

I've been carrying this patch around a while, so I don't have a recent repro case to share. But what I was doing at the time was running "warp delete" with --list-existing against buckets that were already empty with some concurrency (unsure if that's required to trigger the issue).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (Make data collection aggregated throughout #362 per comment here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[klauspost]

lgtm