S2EExecutor: allow throwing exceptions in onStateFork signal

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
S2EExecutor: it is now safe to throw exceptions in onFork()

The patch ensures that the execution is not left in an
inconsistent state.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>

Conflicts:
	klee/lib/Core/Executor.cpp
	qemu/s2e/S2EExecutor.cpp
S2EExecutor: properly finalize states that threw an exception in onStateFork

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>

Conflicts:
	qemu/s2e/S2EExecutionState.cpp
	qemu/s2e/S2EExecutor.cpp