Do not allow access to return value of tasks that ended in exceptions.

[bangerth]

Fixes #15495.

[bangerth]

Fixed as requested.

[tamiko]

This triggers in a number of tests:

base/generate_normal_random_number_01.debug: RUN failed. ------ Partial output:
base/generate_normal_random_number_01.debug: RUN failed. ------ Additional output on stdout/stderr:
--------------------------------------------------------
An error occurred in line <235> of file </jenkins/workspace/dealii-serial_PR-15542/include/deal.II/base/thread_management.h> in function
    RT& dealii::Threads::internal::return_value<RT>::get() [with RT = std::pair<double, double>; dealii::Threads::internal::return_value<RT>::reference_type = std::pair<double, double>&]
The violated condition was: 
    value_is_initialized
Additional information: 
    You cannot read the return value of a thread or task if that value has
    not been set. This happens, for example, if a task or thread threw an
    exception.
--------------------------------------------------------

The following tests FAILED:
	244 - base/generate_normal_random_number_01.debug (Failed)
	3218 - multithreading/thread_local_storage_03.debug (Failed)
	3219 - multithreading/thread_local_storage_04.debug (Failed)
	3233 - multithreading/thread_validity_10.debug (Failed)
	3236 - multithreading/thread_validity_13.debug (Failed)
Errors while running CTest

[bangerth]

Let me reconsider this once #15555 is merged. I believe that some of the failing tests will go away there because the class that triggers the issue is going to go away.

[bangerth]

Seems to work now!