Handle FileExistsException in simpleRename without disconnecting slave

[tsautier]

Problem

Slave crashes when attempting to move a file that already exists at the destination (e.g., during affil-race + site pre). The FileExistsException from the IO operation was propagating uncaught.

Solution

Add specific handling for FileExistsException in the slave's async command handler to prevent crashes.

Changes Made

• Modified exception handling to catch FileExistsException
• Return appropriate error response instead of crashing the slave
• IO errors are now gracefully handled without disconnecting

Fixes: #195

[tsautier]

@mvangoor @hrxcodes for review