ToolchainMissing
class ToolchainMissing(val tool: String, val foundVersion: String?, val message: String) : FilmstripError(source)
An external program this backend drives was not found, or is too old to use.
Distinct from BackendMissing, whose BackendMissing.artifact is a Maven coordinate: nothing a build file can add fixes this one. A Swift caller matching arms with as? gets no compile error for leaving this out, and no Apple backend drives an external program, so it will not reach one.