Fixed plugins folder for MP2
Introduced new OperationSystemException to control application exiting if required. Host can decide to use Environment.Exit or not.
Merge branch 'EXP-TVE3.5-MP1-MP2-FixMP2-Plugin_folder' into EXP-TVE3.5-dev
Merge branch 'EXP-TVE3.5-MP1-MP2-ImprovedOS_Exit' into EXP-TVE3.5-dev