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/state.hh

@@ -33,20 +33,27 @@ typedef enum {
     bsCachedFailure = 8, // steps only
     bsUnsupported = 9,
     bsLogLimitExceeded = 10,
+    bsNarSizeLimitExceeded = 11,
     bsBusy = 100, // not stored
 } BuildStatus;
 
 
-struct RemoteResult : nix::BuildResult
+struct RemoteResult
 {
+    BuildStatus stepStatus = bsAborted;
+    bool canRetry = false; // for bsAborted
+    bool isCached = false; // for bsSucceed
+    bool canCache = false; // for bsFailed
+    std::string errorMsg; // for bsAborted
+
     time_t startTime = 0, stopTime = 0;
     unsigned int overhead = 0;
     nix::Path logFile;
     std::shared_ptr<nix::FSAccessor> accessor;
 
-    bool canRetry()
+    BuildStatus buildStatus()
     {
-        return status == TransientFailure || status == MiscFailure;
+        return stepStatus == bsCachedFailure ? bsFailed : stepStatus;
     }
 };
 
@@ -350,6 +357,8 @@ private:
        tokens are available. */
     nix::TokenServer memoryTokens;
 
+    size_t maxOutputSize = 2ULL << 30;
+
 public:
     State();