proxy: fix error handling in prune scheduling

proxy: fix error handling in prune scheduling

we rely on the jobstate handling to write the error of the worker
into its state file, but we used '?' here in a block which does not
return the error to the block, but to the function/closure instead

so if a prune job failed because of such an '?', we did not write
into the statefile and got a wrong state there

instead use our try_block! macro that wraps the code in a closure

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>