app/instance: wrap internal implementation

This reduces the scope of the fst package, which was growing questionably large.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/instance/errors.go

@@ -0,0 +1,17 @@
+package instance
+
+import (
+	"syscall"
+
+	"git.gensokyo.uk/security/fortify/internal/app"
+	"git.gensokyo.uk/security/fortify/internal/app/internal/setuid"
+)
+
+func PrintRunStateErr(whence int, rs *app.RunState, runErr error) (code int) {
+	switch whence {
+	case ISetuid:
+		return setuid.PrintRunStateErr(rs, runErr)
+	default:
+		panic(syscall.EINVAL)
+	}
+}