(zilch magic): fix ca build error conditions more

core/src/magic.sld

@@ -454,7 +454,7 @@
             (condition-variable-broadcast! build-condvar)
             (when (error-object? e)
               (fprintf (current-error-port) "~S ~A" (thread-name (current-thread)) (error-object-message e)))
-            (quit))
+            (quit #f))
            (lambda ()
             (do-build conn item)))
           (builder quit conn))
@@ -486,6 +486,8 @@
          builder-threads)))
 
      (for-each (lambda (t) (thread-join! t)) builder-threads)
+     (when (eq? pending-count 'error)
+       (error "CA build failed"))
      root-pend)
    (define (drv-resolve-ca drv outputs)
      (if (drv-is-ca drv)