Fail reflink_or_copy if target file already exist

[Vaiz]

This might be a controversial take, so I decided to create a separate PR.

The reflink function fails if the target file exists, whereas reflink_or_copy switches to copying. As a result, instead of saving space when possible, the function creates a full copy. I see two solutions for this problem:

• Make reflink_or_copy act like reflink and return an error if the target file exists.
• Rewrite the function to clone blocks even if the target file exists.

This PR implements the first solution, making reflink_or_copy return an error if the target file exists. I also added a few other unrecoverable errors to the list.

[NobodyXu]

Thanks I think this absolutely makes sense.

reflink_copy shall fallback to std::fs::copy if reflink isn't available, but should not fallback on permission error/dest file already exists/source file not exist.