hot_standby: log error if hot standby fails to apply row

Since commit d2537d9d ("relay: cleanup error handling")
recover_remaining_wals() doesn't log the error it throws -
now callers of this function should catch and log the error.
hot_standby_f() doesn't catch the error so the diagnostic
message is lost if we fail to apply a row in the hot standby
mode. Fix this.

NO_DOC=bug fix
NO_TEST=checked in next commit
NO_CHANGELOG=minor bug in logging