better cleanup execute(callback,...) array members

doc/xschem_man/developer_info.html

@@ -1248,6 +1248,9 @@ C {verilog_timescale.sym} 1050 -100 0 0 {name=s1 timestep="1ns" precision="1ns"
    execute(cmd,last)
    execute(data,last)
    execute(status,last)
+   execute(error,last)
+   execute(exitcode,last)
+   execute(id)
    flat_netlist 0
    fullscreen 0
    gaw_tcp_address {localhost 2020}

src/xschem.tcl

@@ -242,8 +242,8 @@ proc execute_fileevent {id} {
       }
       if {[info exists execute(callback,$id)] && $execute(callback,$id) ne {}} {
         uplevel #0 "eval $execute(callback,$id)"
-        unset execute(callback,$id)
       } 
+      catch {unset execute(callback,$id)} 
       set execute(cmd,last) $execute(cmd,$id)
       set execute(data,last) $execute(data,$id)
       set execute(error,last) $err