Introduce a specialization of `MetaKernel::Error()`
We have introduced a function to filter some of the error messages that we deem should not appear in the notebook.
In kernel.py, there were as a result two calls to self.Error(self._filter_standard_error(...), ...)
This is not very DRY, and when working with !20 (merged) the filtering didn't occur as there were new Error
calls introduced in _stdcin.py
.
As we do not want these errors at all in the notebooks, it is easier and more efficient to redefine Error()
so that this filtering is done all the time.