Better error when calling `get_taped_globals` outside a TapedTask

[mhauru]

julia> using Libtask

julia> Libtask.get_taped_globals(Any)
ERROR: KeyError: key :task_variable not found
Stacktrace:
 [1] getindex
   @ ./iddict.jl:98 [inlined]
 [2] task_local_storage
   @ ./task.jl:293 [inlined]
 [3] get_taped_globals(::Type{Any})
   @ Libtask ~/projects/Libtask.jl/src/copyable_task.jl:21
 [4] top-level scope
   @ REPL[2]:1