Fix why-depends for CA derivations (again)

This has the same goal as b13fd4c58e81b2b2b0d72caa5ce80de861622610,but
achieves it in a different way in order to not break
`nix why-depends --derivation`.

src/libstore/realisation.hh

@@ -93,4 +93,14 @@ struct RealisedPath {
     GENERATE_CMP(RealisedPath, me->raw);
 };
 
+class MissingRealisation : public Error
+{
+public:
+    MissingRealisation(DrvOutput & outputId)
+        : Error( "cannot operate on an output of the "
+                "unbuilt derivation '%s'",
+                outputId.to_string())
+    {}
+};
+
 }