Provide more meaningful output when rockcraft.yaml is invalid
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Issue body
Hi!
I've been trying to run `rockcraft pack` with a valid yaml wrongly indented:
```
name: Indico
summary: Indico rocks
description: Indico OCI images for the Indico charm
version: "1.0"
base: bare
build-base: ubuntu:22.04
license: Apache-2.0
platforms:
amd64:
parts:
indico:
source: plugins
plugin: python
python-packages:
- indico==3.2.0
```
and I'm getting the following output:
```
Logging execution to '/home/arturo/.local/state/rockcraft/log/rockcraft-20230206-173725.594029.log'
rockcraft internal error: ScannerError(None, None, 'mapping values are not allowed here', <yaml.error.Mark object at 0x7fddfe017610>)
Full execution log: '/home/arturo/.local/state/rockcraft/log/rockcraft-20230206-173725.594029.log'
```
I would have expected the output to show the affected line or element.
Thank you
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