Server|Client: Sound deltas emitted from SideDef sections

Added a new delta type (DT_SIDE_SOUND) that is used when a sound delta
is generated for a sound that is emitted by a wall surface.
libdeng2|Loop: Catch exceptions thrown during loop iteration

Uncaught exceptions will not be tolerated in the Qt event loop.
Server: Better app termination handler (qFatal)
Fixed|Server: Uncaught exception when client has wrong game mode

If a client has the wrong game mode, it will be immediately terminated.
This will lead to failure in a subsequent N_SendPacket.

N_SendPacket must catch any exceptions thrown during sending because
the higher-ups have not been designed with exceptions in mind.