Make the output size limit configurable

The maximum output size per build step (as the sum of the NARs of each
output) can be set via hydra.conf, e.g.

  max-output-size = 1000000000

The default is 2 GiB.

Also refactored the build error / status handling a bit.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -41,6 +41,11 @@ State::State()
         }
     }
 
+    {
+        std::string s = hydraConfig["max-output-size"];
+        if (s != "") string2Int(s, maxOutputSize);
+    }
+
     logDir = canonPath(hydraData + "/build-logs");
 }