Return informative exit status from jat__sfinish()

src/jat.sh.skel

     # leftover session and whole log will be merged.
     #
     local fileas    # session id for filing in 'finished' folder
+    local es        # final exit status
     test -d "$JAT__DIR/session" || {
         __jat__show_error "no active session: no $JAT__DIR/session"
         return 2
     }
+    es=$(__jat__final_es)
     __jat__show_sfinish
     __jat__log_event SINFO "finishing session"
     __jat__writelog <<<"finalized: true"
     mv "$JAT__DIR/session" "$JAT__DIR/finished/$fileas"
     rm -f "$JAT__DIR/last"
     ln -s "finished/$fileas" "$JAT__DIR/last"
+    return "$es"
 }
 
 jat__log_error() {
     return 1
 }
 
+__jat__final_es() {
+    #
+    # Print final exit status
+    #
+    # Roughly follows TFKit's semantic; unoffical link:
+    #
+    #     https://github.com/AloisMahdal/shellfu/tree/master/utils/tfkit/doc
+    #
+    test "$(jat__stat serrc)"  -gt 0 && echo 4 && return
+    test "$(jat__stat swarc)"  -gt 0 && echo 3 && return
+    test "$(jat__stat sfailc)" -gt 0 && echo 1 && return
+    echo 0
+}
+
 __jat__log_event() {
     #
     # Pass YAML log event to __jat__writelog()