hydra-queue-runner: Handle exceptions in the dispatcher thread

E.g. "resource unavailable" when creating new threads.

src/hydra-queue-runner/dispatcher.cc

@@ -32,6 +32,8 @@ void State::makeRunnable(Step::ptr step)
 void State::dispatcher()
 {
     while (true) {
+
+        try {
             printMsg(lvlDebug, "dispatcher woken up");
             nrDispatcherWakeups++;
 
@@ -54,6 +56,12 @@ void State::dispatcher()
                 }
                 *dispatcherWakeup_ = false;
             }
+
+        } catch (std::exception & e) {
+            printMsg(lvlError, format("dispatcher: %1%") % e.what());
+            sleep(1);
+        }
+
     }
 
     printMsg(lvlError, "dispatcher exits");