Fix bogus coding in WaitForBackgroundWorkerShutdown().

 Fix bogus coding in WaitForBackgroundWorkerShutdown().
 
 Some conditions resulted in "return" directly out of a PG_TRY block,
 which left the exception stack dangling, and to add insult to injury
 failed to restore the state of set_latch_on_sigusr1.
 
 This is a bug only in 9.5; in HEAD it was accidentally fixed by commit
 db0f6cad4, which removed the surrounding PG_TRY block.  However, I (tgl)
 chose to apply the patch to HEAD as well, because the old coding was
 gratuitously different from WaitForBackgroundWorkerStartup(), and there
 would indeed have been no bug if it were done like that to start with.
 
 Dmitry Ivanov
 
 Discussion: <1637882.WfYN5gPf1A@abook>