[ThinLTO] Fix nondeterministic exit on error.

In the multi-threaded case, if a thread hits an error, we mimick
LLVMContext's behavior of reporting the error and exit-ing. However,
this doesn't cleanly join the other threads, so depending on how fast
the process exits, other threads may report 'terminate called without an
active exception'.

To avoid this non-determinsim, and without introducing a more complicated
design, we just report the error, but not exit early. We do track whether
we hit errors and exit(1) after joining.

Differential Revision: https://reviews.llvm.org/D115574