SL-10190: Introduce LLCoros::saveException() and rethrow().

This mechanism uses a queue of std::exception_ptrs to transport an (otherwise)
uncaught exception from a terminated coroutine to the thread's main fiber. The
main loop calls LLCoros::rethrow() just after giving some cycles to ready
coroutines that frame.

# Conflicts:
#	indra/llcommon/llcoros.cpp
#	indra/llcommon/llcoros.h
#	indra/newview/llappviewer.cpp