Add check for non `.mltemplate` paths when converting `.mlsetup` from json

[notaspirit]

Is your feature request related to a problem? Please describe. If you try to use anything other than .mltemplate as the material in mlsetupbuilder and use that in game the game crashes on save load, but no warnings or errors are given anywhere, not in wkit nor in mlsetupbuilder.

Describe the solution you'd like When trying to convert an .mlsetup from json that has an incorrect or unsupported base material it should throw an error, or at the very least a warning

[seberoth]

I'm against it. Not going to start implementing data validation logic to the json import.