catch final method override errors in dropInvalidMappings

[ix0rai]

it's possible, through renames or matching, that a method ends up mistakenly overriding a final method. this, of course, causes errors in production!

we should implement a catch for this in dropInvalidMappings, likely in a new step (currently existing steps are checking for mappings that don't exist in the jar, then checking for empty mappings) and possibly also on rename.