Rework exception handling from fieldcontainer implementation

[cnowke]

C++:
A : FieldContainer?
{
A(){do();};
void do(){throw std::logic_error("error occurred");};
};

Python:
a = node.A()

leads to ImportError?("Unknown node %s" % self._name)
ImportError?: Unknown node A

My thrown logic_error should be reported back to me.