rest_server: fix TOC/TOU race when listing/fetching E-RABs
The list of E-RAB FSM pids is a snapshot taken at one point in time. By the time we interrogate each erab_fsm process individually, any of them may have already terminated (e.g. bearer released mid-request). The current code fails to generate a response if this happens.
* fetch_erab_info/1: add a pid() clause that wraps erab_list_item/1 in a try/catch, returning 'error' if the process is gone.
* fetch_erab_info/1: catch both exit forms ** `{noproc, _}` raised by gen_statem:call/2 on a monitored pid, and ** the bare noproc atom for other code paths.
* fetch_erab_list/1: switch from lists:map to lists:filtermap and call fetch_erab_info/1 per E-RAB, silently dropping any that died between the snapshot and the per-process interrogation.
