hg-cpython: fix new occuring TypeError dd339191f2dc introduced a minor refactoring of error types that highlighted a fragile error creation mechanism that was in place in the Rust `MixedIndex`. `PyErr::from_instance` also accepts a Python class, as long as it's an `Exception` class. Before the aforementioned commit, we never ran into a case where this duck-typing mechanism failed. We rectify this behavior by doing the instantiation ourselves. Differential Revision: https://phab.mercurial-scm.org/D10764