Return error on out of space

[bwitt]

Fixes https://github.com/aptly-dev/aptly/issues/1474

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Return an error if we run out of disk space. Batch up the sync at the end of writing all files.

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS