Fix error display if execve() in the builder fails

After we've send "\2\n" to the parent, we can't send a serialized
exception anymore. It will show up garbled like

  $ nix-build --store /tmp/nix --expr 'derivation { name = "foo"; system = "x86_64-linux"; builder = "/foo/bar"; }'
  this derivation will be built:
    /nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv
  building '/nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv'...

  ErrorErrorEexecuting '/foo/bar': No such file or directory
  error: builder for '/nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv' failed with exit code 1

src/libstore/build/local-derivation-goal.cc

@@ -1594,6 +1594,8 @@ void LocalDerivationGoal::runChild()
     /* Warning: in the child we should absolutely not make any SQLite
        calls! */
 
+    bool sendException = true;
+
     try { /* child */
 
         commonChildInit(builderOut);
@@ -2050,6 +2052,8 @@ void LocalDerivationGoal::runChild()
         /* Indicate that we managed to set up the build environment. */
         writeFull(STDERR_FILENO, std::string("\2\n"));
 
+        sendException = false;
+
         /* Execute the program.  This should not return. */
         if (drv->isBuiltin()) {
             try {
@@ -2103,10 +2107,13 @@ void LocalDerivationGoal::runChild()
         throw SysError("executing '%1%'", drv->builder);
 
     } catch (Error & e) {
+        if (sendException) {
             writeFull(STDERR_FILENO, "\1\n");
             FdSink sink(STDERR_FILENO);
             sink << e;
             sink.flush();
+        } else
+            std::cerr << e.msg();
         _exit(1);
     }
 }