clean up some cruft from error callstacks

clean up some cruft from error callstacks

before:

  stack traceback:
    [string "text.lua"]:9: in function 'draw'
    [string "edit.lua"]:200: in function 'draw'
    [string "run.lua"]:140: in function 'draw'
    [string "main.lua"]:162: in function <[string "main.lua"]:155>
    [C]: in function 'xpcall'
    [string "app.lua"]:38: in function <[string "app.lua"]:20>
    [C]: in function 'xpcall'
    [love "boot.lua"]:370: in function <[love "boot.lua"]:337>

after:

  stack traceback:
    text.lua:9: in function 'draw'
    edit.lua:200: in function 'draw'
    run.lua:140: in function 'draw'
    main.lua:162: in function <[string "main.lua"]:155>
    [C]: in function 'xpcall'
    app.lua:38: in function <[string "app.lua"]:20>
    [C]: in function 'xpcall'
    [love "boot.lua"]:370: in function <[love "boot.lua"]:337>