fix(task) #35 FileFetchTask no longer ignores write failures

[njoubert-cleverage]

Description

Fixes #35.

FileFetchTask caught and discarded the FilesystemException thrown when writing to the destination storage: the process succeeded, the file name was output as if the copy had worked and, with remove_source: true, the source file was deleted although it was never copied.

• FileFetchTask::doFileCopy(): a write failure is no longer caught, so the task fails and its error_strategy applies; the source is deleted only after a successful write; the read stream is always closed (finally)
• FileFetchTaskTest (new): a successful copy with remove_source deletes the source and outputs the file name; a write failure throws UnableToWriteFile, keeps the source and outputs nothing (fails without the fix)
• file_fetch_task.md: document the write failure behaviour

Checked with demo.flysystem.fetch_write_failure in process-bundle-demo: the process now fails with Unable to write file ... Is a directory and the source file is kept (it succeeded and deleted the source before).

Requirements

• Documentation updates
  • Reference
  • Changelog
• Unit tests

Breaking changes

A write failure now fails the task (with the default stop strategy, the process fails) instead of being silently ignored.

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