Improve test coverage

[pydanny]

Our current test coverage on September 30th is 52%:

Name                            Stmts   Miss  Cover   Missing
-------------------------------------------------------------
src/dj_notebook/__init__.py        13      3    77%   28-30
src/dj_notebook/shell_plus.py      44     26    41%   24-38, 43-51, 55, 60-67, 80-85, 89-91
tests/__init__.py                   0      0   100%
tests/test_dj_notebook.py           4      0   100%
-------------------------------------------------------------
TOTAL                              61     29    52%

Any improvement that increases the total percentage by at least 1% qualifies for hacktoberfest.