Install may die on a theme init error

If the theme errors or is removed between installs/updates, Composer will die on the following error:

> cannot_instantiate_theme: pohjola, pohjola_theme

...because old-school Textpattern uses explicit process termination instead of Exceptions.

We should either:
- Try to simulate Textpattern admin-side interface, skipping theme.
- Default to core theme.
