Add check_only parameter to get_closure handler

Add check_only parameter to get_closure handler

`get_closure` handlers are called to check whether an object is
callable, and to actually get the closure, respectively.  The behavior
of the handler might differ for these two cases, particularly the
handler may throw in the latter case, but should not in the former.

Therefore we add a `check_only` parameter, to be able to distinguish
the desired purpose.