Enhance Isocline PSE viewer to also work when NaNs are generated or some simulation failed.

This visualizer is not available if any operation finished in 'ERROR'.
If it's because the TimeSeries is full of NaNs, it's useful information as well and should be displayed (by default, NaNs are displayed in white using 'imshow').
If it's for any other reason, simulations finished in error can be set to have a particular color (eg, black) to know at which combination of parameters the problem occurred.