[Rd] tryCatch without handler in QC.R

[suh@rto_@@ggo@o m@iii@g oii y@hoo@com]

The following is among those added by r77792 to src/library/tools/R/QC.R in R devel.
                bad <- tryCatch(check_url_db(udb))

The 'tryCatch' has no handler. It does nothing. Error would still stop execution.