Don't crash when copying realisations to a non-ca remote

Rather throw a proper exception, and catch&log it on the client side

src/libstore/globals.cc

@@ -165,10 +165,15 @@ bool Settings::isExperimentalFeatureEnabled(const std::string & name)
     return std::find(f.begin(), f.end(), name) != f.end();
 }
 
+MissingExperimentalFeature::MissingExperimentalFeature(std::string feature)
+    : Error("experimental Nix feature '%1%' is disabled; use '--experimental-features %1%' to override", feature)
+    , missingFeature(feature)
+    {}
+
 void Settings::requireExperimentalFeature(const std::string & name)
 {
     if (!isExperimentalFeatureEnabled(name))
-        throw Error("experimental Nix feature '%1%' is disabled; use '--experimental-features %1%' to override", name);
+        throw MissingExperimentalFeature(name);
 }
 
 bool Settings::isWSL1()

src/libstore/globals.hh

@@ -45,6 +45,16 @@ struct PluginFilesSetting : public BaseSetting<Paths>
     void set(const std::string & str, bool append = false) override;
 };
 
+/* MakeError(MissingExperimentalFeature, Error); */
+class MissingExperimentalFeature: public Error
+{
+public:
+    std::string missingFeature;
+
+    MissingExperimentalFeature(std::string feature);
+    virtual const char* sname() const override { return "MissingExperimentalFeature"; }
+};
+
 class Settings : public Config {
 
     unsigned int getDefaultCores();

src/libstore/local-store.cc

@@ -655,6 +655,7 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
 
 void LocalStore::registerDrvOutput(const Realisation & info)
 {
+    settings.requireExperimentalFeature("ca-derivations");
     auto state(_state.lock());
     retrySQLite<void>([&]() {
         state->stmts->RegisterRealisedOutput.use()

src/libstore/store-api.cc

@@ -794,9 +794,19 @@ std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStor
             realisations.insert(*realisation);
     }
     auto pathsMap = copyPaths(srcStore, dstStore, storePaths, repair, checkSigs, substitute);
+    try {
         for (auto& realisation : realisations) {
             dstStore->registerDrvOutput(realisation);
         }
+    } catch (MissingExperimentalFeature & e) {
+        // Don't fail if the remote doesn't support CA derivations is it might
+        // not be whithin our control to change that, and we might still want
+        // to at least copy the output paths.
+        if (e.missingFeature == "ca-derivations")
+            ignoreException();
+        else
+            throw;
+    }
 
     return pathsMap;
 }