Changes

Changes from Mercurial (hg https://isabelle.in.tum.de/repos/isabelle/ default)

Summary

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