Previously, when an exception was raised from
'handle-build-event' (e.g., a "database is locked" error), we'd throw,
thereby leaving PORT open and we'd never read from it again.  Thus, the
corresponding 'guix-daemon' process would eventually get stuck in a
'write' call to that socket, and its build processes would stall.

* src/cuirass/base.scm (exception-reporter): New procedure.
(spawn-builds): Use it.