reduced effectiveness of raising fatal errors (RMGLog::fatal) in multithreaded runs

[ManuelHu]

• RMGLog::Out(RMGLog::fatal, ...) is does not terminate the application when called on a worker thread.
• G4RunManager::GetRunManager()->AbortRun() is also of no use to abort the whole run, if used in any BeginOfRunAction
• ...also it only works only for the local thread, and not for all threads together.
• --> there is no clean way to check prerequisites on a worker, and to abort if they are not met

see #198 for a related issue

[ManuelHu]

reworded: this is not about fatal errors raised in MT mode, but about our own raising of fatal errors in MT mode, which might not do what we want and expect