Commit breaks test for ftolerance

[freifrauvonbleifrei]

... although it really should not!

After this commit here
fca872c
the ftolerance tests somehow suddenly end before the RUN_FIRST signal is finished.

However, the commit only adds a runtime error in DFG, a function in test_helper and calls to that function in some tests. Not the ftolerance test.

To really show this, I have added an error to the test

DisCoTec/distributedcombigrid/tests/test_ftolerance.cpp

Line 431 in 2c7bef9

BOOST_TEST_ERROR("worker never cleanly exits! " + std::to_string(signal));

which is never thrown!

[freifrauvonbleifrei]

Here you can see the test's output:
https://simsgs.informatik.uni-stuttgart.de/jenkins/view/DisCoTec/job/DisCoTec-third-level-advection/104/console