vendir sync --json swallows error

**What steps did you take:**
- have a vendir.yaml that produces an error, eg
```
vendir: Error: Syncing directory 'foo':
Copying another directory contents into directory 'bar': lstat ../src/foo: no such file or directory
```
- run `vendir sync`: produces a nice error
- run `vendir sync --json`

**What happened:**
produces no error, but does exit with code 1

**What did you expect:**
error message like without `--json`, in json and/or to stderr

**Environment:**

- vendir version (execute `vendir --version`): vendir version 0.38.0
- OS (e.g. from `/etc/os-release`): debian bookworm

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Reproduce the issue with a vendir.yaml that references the missing ../src/foo path, then compare vendir sync with vendir sync --json. Trace the sync command's JSON error handling and add coverage for the failed sync case. Done means the JSON mode reports the error in JSON and/or on stderr while retaining exit code 1.

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