Simplify AtomicFileWriter and use clearer temporary file names

[Vlatombe]

• Uses a more readable name for the temporary file, derived from the target path
• Streamline the fallback logic for atomic move, and just let the error bubble up otherwise rather than logging then throwing which was leading to duplicate stacktraces in the system logs

Related to #2548 (comment)

See JENKINS-XXXXX.

Testing done

Proposed changelog entries

• human-readable text

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N/A

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[jglick]

Would be nice if this were logged only once per session. Or if subsequent calls to commit just went straight to non-atomic move.