Make Env::AppBundle constructor exception safe

When the Env::AppBundle constructor throws an exception
it still needs to run ~AppBundle(), otherwise the JobQueue
isn't properly shut down.  Specifically the  JobQueue
can destruct without waiting on outstanding jobs in the
queue.

This change ensures that if Env::AppBundle constructor
throws, Env::AppBundle::~AppBundle() runs.

This fixes the unit test crash exposed by PR #3047.

src/test/jtx/Env_test.cpp

@@ -749,6 +749,22 @@ public:
         }
     }
 
+    void testExceptionalShutdown()
+    {
+        except(
+            [this]
+            {
+                jtx::Env env {*this,
+                              jtx::envconfig([](std::unique_ptr<Config> cfg)
+                              {
+                                  (*cfg).deprecatedClearSection("port_rpc");
+                                  return cfg;
+                              })};
+            }
+        );
+        pass();
+    }
+
     void
     run() override
     {
@@ -771,6 +787,7 @@ public:
         testResignSigned();
         testSignAndSubmit();
         testFeatures();
+        testExceptionalShutdown();
     }
 };