backend: use FileHandler for backend.log
    
    WatchedFileHandler isn't appropriate choice, that's not what we actually
    need.  But more importantly, it even caused problems on build cancel:
    
    1. user requested build cancel
    2. user requested build delete very quickly
    3. the build delete action was processed first, and build was removed
       (including the backend.log)
    4. cancel request is processed
    5. background worker tracebacks on the first attempt to log into the
       backend.log file because we don't only remove the log file, but whole
       directory.
    
    Where WatchedFileHandler failed before to re-create the file, we simply
    ignore the unlink()ed file problem and keep writing into the opened file
    descriptor.
    
    As a consequence, because of unexpected error in logging, we did not
    even release the allocated VM before (so tickets were kept open, and
    resources UP, and unused).