Immutable DB: simulate failed file handle closes

What does it mean to fail to close a file handle? Will it remain open, can it still be used, should we try to close it again?

It might be that closing the handle forces a flush of the last write(s), which causes the failure.

Read the manual (OS docs, Posix manual, Win32 manual, etc).

We can easily simulate this using the `Error` `HasFS`.

Start by reading the relevant OS, POSIX, and Win32 manuals about failed file-handle closes, including whether the handle remains open, can be reused, or should be closed again. Then inspect the `Error` `HasFS` implementation to determine how close failures can be simulated. Done means the expected close behavior is established and the simulation covers the failure, including any flush-related error.

Written by the indexing model from the issue text.