Increase test coverage

[rocodes]

Filing to ensure that we set aside time for further test coverage on sd-export.

From the current tip of main:

----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name                                  Stmts   Miss  Cover   Missing
-------------------------------------------------------------------
securedrop_export/__init__.py             1      0   100%
securedrop_export/disk/__init__.py        0      0   100%
securedrop_export/disk/actions.py       147     71    52%   26, 56-57, 89, 104-133, 137-158, 167-192, 197, 200-201, 209-212, 220-229
securedrop_export/entrypoint.py          50     50     0%   1-80
securedrop_export/exceptions.py          27      1    96%   49
securedrop_export/export.py              75      4    95%   54-56, 143
securedrop_export/main.py                29     22    24%   13-39
securedrop_export/print/__init__.py       0      0   100%
securedrop_export/print/actions.py      144     71    51%   29, 34-51, 54-77, 84-85, 100-101, 156-157, 160-167, 189-201, 212-215, 220, 223-226, 231, 234-235
securedrop_export/utils.py               61     30    51%   22-48, 117, 126-138, 145-149
-------------------------------------------------------------------
TOTAL                                   534    249    53%
Coverage HTML written to dir htmlcov