Improve cloud-init error messages

[ricab]

Fix #3693.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.90%. Comparing base (dd7cfb0) to head (3f0c243). Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/client/cli/cmd/launch.cpp	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3705 +/- ## ========================================== - Coverage 88.94% 88.90% -0.04% ========================================== Files 254 254 Lines 14276 14279 +3 ========================================== - Hits 12698 12695 -3 - Misses 1578 1584 +6 ``` | [Flag](https://app.codecov.io/gh/canonical/multipass/pull/3705/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/canonical/multipass/pull/3705/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | `88.90% <75.00%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical#carryforward-flags-in-the-pull-request-comment) to find out more.

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