We only need to parse the libraries for the Sim Model Editor dialog
(so that we can determine the models to put in the popup). Doing it
for netlisting just opens us up to incorrectly parsing the SPICE, and
returns no value.
This also means that a SIM_LIB_MGR manages a single library, and
never multiple libraries.
This also allows the tidying-up of some error reporting structures to
better inform the user what went wrong.
Fixes https://gitlab.com/kicad/code/kicad/issues/13431
Jeff Young
Seth Hillbrand