Copy world should notify about file system errors

[mk-pmb]

Operating System

No response

Description of bug

"Copy world" conceals file system errors. The only feedback is that, after a while, the new world appears in the worlds list. There is no warning that in the copy, some regions are missing.

Steps to reproduce

Use Ubuntu 22.04, openjdk 17, MultiMC 5. (Cannot copy version details, see #4810.)

Prepare to provoke a file system error, e.g. by having insufficient free disk space. Copy a world. Wait until it shows up in the worlds list. Start the instance in offline mode. Join the copied world. Dangle in black void for suspiciously long time. Once the area becomes visible, discover that a drained lava pool is no longer drained. (The chunk has been regenerated because the file system had failed to copy some region files.)

Suspected cause

No response

This issue is unique

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