libstore: keep Goal errors as unique_ptrs

Error is pretty large, and most goals do not fail. this alone more than
halves the size of Goal on x86_64-linux, from 720 bytes down to 344. in
derived classes the difference is not as dramatic, but even the largest
derived class (`LocalDerivationGoal`) loses almost 20% of its footprint

Change-Id: Ifda8f94c81b6566eeb3e52d55d9796ec40c7bce8

src/libstore/build/entry-points.cc

@@ -22,7 +22,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
             if (ex)
                 logError(i->ex->info());
             else
-                ex = std::move(i->ex);
+                ex = std::move(*i->ex);
         }
         if (i->exitCode != Goal::ecSuccess) {
             if (auto i2 = dynamic_cast<DerivationGoal *>(i.get()))

src/libstore/build/goal.cc

@@ -78,7 +78,7 @@ void Goal::amDone(ExitCode result, std::optional<Error> ex)
         if (!waiters.empty())
             logError(ex->info());
         else
-            this->ex = std::move(*ex);
+            this->ex = std::make_unique<Error>(std::move(*ex));
     }
 
     for (auto & i : waiters) {

src/libstore/build/goal.hh

@@ -121,7 +121,7 @@ public:
     /**
      * Exception containing an error message, if any.
      */
-    std::optional<Error> ex;
+    std::unique_ptr<Error> ex;
 
     Goal(Worker & worker, DerivedPath path)
         : worker(worker)