capture close() err on subnet file save

[ahmetb]

Description

The OS may accept write()s but may fail on close(), which is currently ignored. Because it's ignored, it won't be logged anywhere either. (I'm sending this PR because I'm seeing missing subnet.env file on disk, but there's no Failed to write subnet file).

Todos

• [ ] Tests: N/A
• [ ] Documentation: N/A
• [ ] Release note: N/A

Release Note

None required

[ahmetb]

@manuelbuil please take a look

[manuelbuil]

That change looks good. Given that the buffer will not get superbig, I think this approach makes more sense. Thanks @ahmetb