Comment 1 for bug 1478213

Some interesting/peculiar findings:

* The core problem is that, for some reason, the exception object which we throw is not caught by catch(...) clauses!
* Using a non-empty message in std::runtime_error or another exception type (e.g. std::exception), avoids the failures
* If in mir::GLibMainLoop::enqueue() we replace the invocation of the 'action' function object (which contains a throw) with a direct throw, the exception is caught properly.