Allow hooke.ui to load with missing hooke.license submodule.

Allow hooke.ui to load with missing hooke.license submodule.

This is like similar code in hooke/__init__.py, and allows you to run
Hooke from a snapshot without generating the license.py file.

hooke/license.py is generated as needed by update_copyright.py, so
it's not tracked by the version control system.  However, running
update_copyright.py requires a working Mercurial repository, which
users may not have if they installed from a VCS tarball, etc.  This
code allows those users to run Hooke from such snapshots.

Official releases should bundle a generates license.py file, so in
that case this code is superfluous.