KeyError in indicate_errors for steady fields in combined steady-unsteady fields

[ddundo]

Since #114 and different solution dictionary structures for steady and unsteady fields, L160 and L162 in indicate_errors raise a KeyError. It happens when we have a combination of steady and unsteady fields, so they raise a KeyError for the "steady" fields which do not have the FWD_OLD key in their solutions dictionary.

I guess I'm the only one with this problem at the moment so I fixed it locally and we can address it properly in the future alongside #55

[jwallwork23]

Thanks for reporting this @ddundo. I do intend to get back to finishing your multiple variables issue but there have been various other changes blocking progress along the way. Currently, I'm waiting for #113 to go through. (Feel free to pick it up for review if you like.)

[ddundo]

This is actually not relevant since lagged fields will be extracted (see #153), so I'll close