_createFile opened the cache file with O_RDWR but not O_CREATE, relying
on the file already existing. When the file had been removed underneath
rclone - either by _checkObject dropping a stale entry during open, or by
external deletion - the open failed and surfaced a hard "IO error" to the
application instead of recreating the file. Add O_CREATE so the cache
self-heals in that case.
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