Converting a CLR value to Python can throw after the call has
succeeded, eg if an encoder's TryEncode throws, or if calling
enumerator.Current throws. Previously the exception was never
converted to a Python exception making it uncatchable even
instead, so Python code could not catch it, not even using
except BaseException

- MethodBinder.Invoke guarded only the invocation, and converted the
  result and out parameters after its try/catch
- Iterator.tp_iternext guarded MoveNext but not Current or the
  conversion of the item
- ArrayObject indexing converted items with no guard at all

Each now reports the failure through Exceptions.SetError, as the
existing catch already does for the call itself. Property and field
getters already convert inside their try and are unchanged.