hg-cpython: fix new occuring TypeError...
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
