[mlir][python] Fail fast when nanobind is missing for Python bindings (#174649)

Remove problematic `return()` statements from
mlir_configure_python_dev_packages macro and convert nanobind detection
to FATAL_ERROR.

Using `return()` in a CMake macro is problematic because it returns from
the calling scope, not the macro itself, causing the parent
CMakeLists.txt to silently skip subsequent configuration steps. This
leads to confusing build failures when Python bindings are enabled but
nanobind is missing.

With this change, users get an immediate error with clear instructions
to install nanobind or set nanobind_DIR, rather than silent
configuration failures.

GitOrigin-RevId: 3c848122329e0c397dd10d9beb800d35f1dd3328