Add section on raising errors from failing model runs

[tomalrussell]

Add guidance to nismod.github.io and follow up in each of the models:

If the e.g. transport and energy models fail, then errors are not caught in the runtime
environment and the programme exits silently. Errors should be raised, with messages passed to
stderr so that our subprocess recognises that the programme has exited abnormally.

We need to decide on what to do in the following cases:

• model is infeasible - it hasn't been possible to solve the problem
• data is missing so the model cannot run
• inputs are out of range or invalid (e.g. negative efficiency or cost)