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/libcmd/installables.cc

@@ -931,10 +931,7 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Instal
                                 DrvOutput outputId { *outputHash, output };
                                 auto realisation = store->queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
+                                    throw MissingRealisation(outputId);
-                                        "cannot operate on an output of the "
-                                        "unbuilt derivation '%s'",
-                                        outputId.to_string());
                                 outputs.insert_or_assign(output, realisation->outPath);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

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())
+    {}
+};
+
 }

src/libstore/remote-store.cc

@@ -879,10 +879,7 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
                                 auto realisation =
                                     queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
+                                    throw MissingRealisation(outputId);
-                                        "cannot operate on an output of unbuilt "
-                                        "content-addressed derivation '%s'",
-                                        outputId.to_string());
                                 res.builtOutputs.emplace(realisation->id, *realisation);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

src/nix/why-depends.cc

@@ -95,19 +95,25 @@ struct CmdWhyDepends : SourceExprCommand
          * to build.
          */
         auto dependency = parseInstallable(store, _dependency);
-        auto dependencyPath = Installable::toStorePath(getEvalStore(), store, Realise::Derivation, operateOn, dependency);
+        auto optDependencyPath = [&]() -> std::optional<StorePath> {
-        auto dependencyPathHash = dependencyPath.hashPart();
+            try {
+                return {Installable::toStorePath(getEvalStore(), store, Realise::Derivation, operateOn, dependency)};
+            } catch (MissingRealisation &) {
+                return std::nullopt;
+            }
+        }();
 
         StorePathSet closure;
         store->computeFSClosure({packagePath}, closure, false, false);
 
-        if (!closure.count(dependencyPath)) {
+        if (!optDependencyPath.has_value() || !closure.count(*optDependencyPath)) {
-            printError("'%s' does not depend on '%s'",
+            printError("'%s' does not depend on '%s'", package->what(), dependency->what());
-                store->printStorePath(packagePath),
-                store->printStorePath(dependencyPath));
             return;
         }
 
+        auto dependencyPath = *optDependencyPath;
+        auto dependencyPathHash = dependencyPath.hashPart();
+
         stopProgressBar(); // FIXME
 
         auto accessor = store->getFSAccessor();