Unexpected error message when adding parts

[kris-brown]

g = Graph 
add_part!(g, :V)

this is a user error (should have been Graph()) results in a StackOverflowError in contrast to things like rem_part! (which results in a method not found error). If there's any way to avoid this it would make things more user friendly!

[epatters]

This should be fixable by appropriately restricting the types of add_part(s)!.