Changes

Summary

1. renamed lemmas
2. build manager: echo error messages to server output;
3. omit showing previous failures for user builds;
4. always handle interrupted jobs;
5. add cluster/hosts configurations to build manager: allows running jobs in parallel on distinct hardware;
6. clarified context: operations now in build process;
7. clarified: add explicit build process;
8. remove unnecessary subdir;
9. tuned;