[better_errors] Make it explicit that debug_info is not None.

[copybara-service]

[better_errors] Make it explicit that debug_info is not None.

Now all internal uses of lu.wrap_init and core.Jaxpr are with actual
debug info. This enables us to clean up the type declarations and
to remove the checks whether debug_info is present.

For usage outside of the JAX internals, we change
jax.extend.linear_util.wrap_init to be usable without debug_info,
for temporary backwards compatibility. We emit a deprecation
warning and fill-in some fake debugging info.

See #26480 for more details.