Fix double throw of captured exception in thread group. (#606)

Fixes #603.

* Clear exception pointer in exception_state helper class after move of state.
* Rename exception_state::get() method to reset().
* Call std::terminate() if task_group is destroyed before tasks are collected with wait().
* Do not attempt to collect tasks in destructor for task_group.
* Do not attempt to rethrow exception in destructor for exception_state.
* Add unit test to verify correct exception behaviour when a task_group is runs and waits on a series of tasks.
* Add unit test for terminate behaviour as above.

Code quality fix ups:
* Remove unused warning variable warning in threading exception tests.
* Address if-statement spacing in threading.hpp.
* Use ARB_HAVE_MPI in execution_context.cpp instead of introducing a dependency on generated version header via feature macro ARB_MPI_ENABLED.