Print the error when shelling into the build instance with `--debug`
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Current evaluation
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Issue body
### What needs to get done
The `--debug` flag shells into the build instance in case of an error during the lifecycle, but doesn't show the actual error:
```
❯ snapcraft pack --debug
'override-build' in part 'xkcd' failed with code 1.
Launching shell on build environment...
snapcraft-purugupta99-xkcd-scraper-amd64-33817771 ../project#
exit
'override-build' in part 'xkcd' failed with code 1.
Detailed information:
:: + craftctl default
:: + install -Dm755 src/scrape.py /root/parts/xkcd/install/bin/xkcd_scraper
:: install: cannot stat 'src/scrape.py': No such file or directory
Recommended resolution: Review the scriptlet and make sure it's correct.
For more information, check out: https://documentation.ubuntu.com/snapcraft/8.11.1/reference/plugins/
Failed to run snapcraft in instance
Full execution log: '/home/sergio.schvezov@canonical.com/.local/state/snapcraft/log/snapcraft-20250819-092945.344462.log'
```
### Why it needs to get done
Users will have a better chance of fixing issues if they know what the issue is, presumably.
Evaluation history
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