Backport #26634 by @delvh
Previously, `err` was defined above, checked for `err == nil` and used
nowhere else.
Hence, the result of `convertMinioErr` would always be `nil`.
This leads to a NPE further down the line.
That is not intentional, it should convert the error of the most recent
operation, not one of its predecessors.
Found through
https://discord.com/channels/322538954119184384/322538954119184384/1143185780206993550.
Co-authored-by: delvh <dev.lh@web.de>
a4b14638b5