[BUG] If HDF5 store is not set before simulation starts, an entire simulation is run before failing to save the results.

[myxie]

Describe the bug
A clear and concise description of what the bug is.
A simulation can spend hours running successfully, only to fail at the final output.

INFO:topsim.core.simulation:Simulation Finished @ 127474
Traceback (most recent call last):
  File "/home/rbunney/job_outputs/../github/experiments/simulator_demonstration_experiments/topsim_test_simulations/playground_scalability_new.py", line 82, in <module>
    e.run()
  File "/home/rbunney/github/topsim/topsim/utils/experiment.py", line 172, in run
    s.start()
  File "/home/rbunney/github/topsim/topsim/core/simulation.py", line 272, in start
    if self.to_file and self._hdf5_store is not None:
AttributeError: 'Simulation' object has no attribute '_hdf5_store'

This is caused by an issue with the HDF5 result file pathname that has been passed to the Simulation constructor to be unavailable (either open by another program, or the file does not exist).

To Reproduce
Steps to reproduce the behavior:

Run multiple Simulations at once using the same hdf5_path parameter.

Expected behavior
A clear and concise description of what you expected to happen.

There's not currently an expected behaviour; however, I think we should just exit that simulation immediately rather than run the simulation without an ability to save the results.

Additional context
Add any other context about the problem here.