fix call-with-transaction error (disconnected rollback)

Also, clarify in docs that break causes disconnect.

collects/db/private/generic/functions.rkt

@@ -291,7 +291,8 @@
                      (with-handlers ([exn?
                                       (lambda (e2)
                                         (error/exn-in-rollback 'call-with-transaction e1 e2))])
-                       (send c end-transaction '|call-with-transaction (rollback)| 'rollback #t))
+                       (when (send c connected?)
+                         (send c end-transaction '|call-with-transaction (rollback)| 'rollback #t)))
                      (raise e1))])
     (begin0 (call-with-continuation-barrier proc)
       (send c end-transaction '|call-with-transaction (commit)| 'commit #t))))

collects/db/scribblings/query.scrbl

@@ -49,6 +49,8 @@ disconnected:
 @item{changing communication settings, such as changing the
   connection's character encoding}
 @item{communication failures and internal errors in the library}
+@item{a @seclink["breakhandler" #:doc '(lib "scribblings/reference/reference.scrbl")]{break} 
+  occurring during a connection operation}
 ]
 See @secref["transactions"] for information on how errors can affect
 the transaction status.