Gracefully handle outdated .ninja_log during '-t recompact'

When we explicitly unlink the file we should return LOAD_NOT_FOUND instead of LOAD_SUCCESS

src/build_log.cc

@@ -293,7 +293,7 @@ LoadStatus BuildLog::Load(const string& path, string* err) {
         unlink(path.c_str());
         // Don't report this as a failure.  An empty build log will cause
         // us to rebuild the outputs anyway.
-        return LOAD_SUCCESS;
+        return LOAD_NOT_FOUND;
       }
     }