Prevent Tcl from crashing on unknown command -

i'm doing eval on content of file. file made out of labels parse. each line has label, , have proc defined each label, eval succeeds. however, users add new labels , eval command fails, because of unknown command.

is there way prevent tcl crashing when trying eval unknown command?
ideally, should allow me substitute own defined behaviour - such priting error , continuing eval.

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edit:

unfortunately, can use tcl 8.4.
tried doing following, as suggested here:

proc handle_unknown_label {cmd args} { ... } 

and then:

rename unknown _old_system_unknown rename handle_unknown_label unknown  catch {set ret [eval $x]} err  rename unknown handle_unknown_label rename _old_system_unknown unknown 

but still same behavior eval, , prints following errors:

procedure unknown protected proc , not renamed
procedure unknown protected proc , not overriden
procedure unknown protected proc , not renamed
procedure unknown protected proc , not overriden

i think i'm stating obvious, eval'ing file dangerous: user can embed [exec whatever wish] such file.

much better read "labels" , use giantic switch or hash map or whatever execute predetermined command. catching non-existing "labels" becomes non-issue.