Error handling for insert

[ramcdougal]

Overview of the feature

Attempting to do this should raise a meaningful error message instead of segfaulting:

>>> from neuron import n
>>> soma = n.Section("soma")
>>> n.hh.insert(soma(0.5))

segfault occurs in mac and linux. Haven't tested Windows.

The underlying issue here is that the attempt to iterate through segments with no inserted mechanisms is failing:

>>> from neuron import n
>>> soma = n.Section("soma")
>>> for mech in soma(0.5): break
... 
zsh: segmentation fault  python