Implement N4088: Task Region R3
Cleaning up task_region
Fix comment
Addressing open issues
Changing exception handling to conform to the proposal
Making sure using non-active task_region_handle causes exceptions
Merge branch 'master' into n4088

Conflicts:
	docs/hpx.idx
Updating for removed exception_list::get_error_count()
Merge pull request #1187 from STEllAR-GROUP/n4088

Implement N4088: Task Region R3