Fail loudly when solver step converges

[ahmadia]

Proteus should raise an error instead of silently quitting

[cekees]

OK, I changed the title @leajenkins . Looking at this now, I think he meant that on an iteration failure, proteus will just log that the step fails and then proceed to shutdown without anymore fanfare.

The options I was talking about on the call were the ones that allow the solver to continue running:

nonlinearSolverConvergenceTest = 'rits'

which means converge based on "residual" or "iterations".

[adimako]

@cekees, we this is a straightforward one. Worth capitalising - adding exclamation marks to messages, but not change the sequence for solver failure and shut down