Restricts worker terminated message to error conditions. rmprocs() waits...

... for workers to exit.

Addresses issues raised in https://github.com/JuliaLang/julia/commit/68256c137ed0ebd52892a1b1e162d45576b45149#commitcomment-3665073

rmprocs also has an optional timeout parameter.

Also treating all exceptions the same in the socket event loop for the worker connection.