Currently the cache updaters aren't exception safe
yet they are intended to be.
Instead of allowing exceptions from
`external_updater::execute` escape `row_cache::update`,
abort using `on_fatal_internal_error`.
Future changes should harden all `execute` implementations
to effectively make them `noexcept`, then the pure virtual
definition can be made `noexcept` to cement that.
\Fixes scylladb/scylladb#15576
\Closes scylladb/scylladb#15577
* github.com:scylladb/scylladb:
row_cache: abort on exteral_updater::execute errors
row_cache: do_update: simplify _prev_snapshot_pos setup
(cherry picked from commit 4a0f16474f)
Closesscylladb/scylladb#16256
6d7919041b