Fixed bug that cause Service exceptions to cause aborts

If a Service threw an exception during the ending of a processing transition (e.g. end of Event), the exception would cause an abort rather than being propagated as expected.
The problem was the Service signal was generated during the destructor of a 'guard' class instance. In C++11 those destructors are now implicitly noexcept(true) which caused the abort.
The destructor were changed to noexcept(false) and a new method 'allowThrow()' was added which is used to avoid throwing an exception while an exception is being propagated out
of the function which uses the guard.

This fix was already introduced to CMSSW_7_0_X but given the large changes between the two releases this had to be reimplemented by hand.