Overlay, OverlayClient: add PID and process name diagnostics to 'Dead client detected' notice.

I'm going to use this to diagnose some regular client freezes I've been experiencing on Windows.

My own client is running with this patched in as we speak.  I think the culprit of the freezes/hangs are Spotify, but I'll need to run with the patch some more to be sure.

I think these extra diagnostics make sense to have upstream as well.  Thoughts?
Right now, this is the absolute path to the executable.  We might just want to use the basename...