Do not swallow StopIteration during initialization of previous_current_next

Advancing an iterator twice when we know there are two items (the two
Nones at the start and at the end) will never fail except if `items` is
some funny StopIteration-raising generator.  There's no point in
swallowing this exception.
Test recursetree tag parsing failure
Test that invalid node ordering is detected in cache_tree_children
Stop using list comprehensions where generator expressions suffice
Add a test for tree_info and fix typos in the docstring
Run tests under coverage (if it is installed)
Test that forms generated from models using TreeForeignKey etc. use the expected field types