Fix: [Network] don't desync if client leaves before you finish downloading map

When you are downloading a map, all the commands are queued up
for you. Clients joining/leaving is done by the network protocol,
and as such are processed immediately. This means that by the
time you are processing the commands, a client that triggered
it, might already have left.

So, all commands that do something with ClientID, shouldn't
error on an invalid ClientID when DC_EXEC is set, but
gracefully handle the command anyway, to make sure the
game-state is kept in sync with all the clients that did
execute the DoCommand while the now-gone client was still
there.

Additionally, in the small chance a client disconnects between
the server validating a DoCommand and the command being
executed, also just process the command as if the client was
still there. Otherwise, lag or latency can cause clients that
did not receive the disconnect yet to desync.