Unhelpful error if altair not installed

[ADBond]

I used the below option but always gets this error
nam alt is not defined.

Tried installing alt but still same issue

Originally posted by @aamir-rj in #2459

If users find themselves trying to run Splink in an environment where altair is not installed for some reason, they are left, on trying to import Splink, with the unclear error: NameError: name 'alt' is not defined.

This originates from splink.internals.charts as we create a type alias ChartReturnType for use both in that module and elsewhere for type hinting.

We should perhaps rework this a little, and maybe convert this to a ModuleNotFoundError to make the remedy clearer.