Better guard against exceptions in the build event handlers

guix-data-service/web/build-server/controller.scm

@@ -144,7 +144,20 @@
             (with-postgresql-connection
              "build-event-handler-conn"
              (lambda (conn)
-               (handler conn ids)))))))))
+               (with-exception-handler
+                   (lambda (exn)
+                     (simple-format
+                      (current-error-port)
+                      "exception in build event handler: ~A\n"
+                      exn))
+                 (lambda ()
+                   (with-throw-handler #t
+                     (lambda ()
+                       (handler conn ids))
+                     (lambda _
+                       (display (backtrace) (current-error-port))
+                       (display "\n" (current-error-port)))))
+                 #:unwind? #t)))))))))
 
   (define (handle-derivation-events conn items)
     (if (null? items)