Increase test coverage

[jsignell]

The test coverage in this project is pretty good, but as #587 made clear, there are some areas that are less well-tested.

Here is the output of pytest --cov pystac_client --cov-report term-missing at 7d3be2f

Name                                 Stmts   Miss  Cover   Missing
------------------------------------------------------------------
pystac_client/__init__.py                7      0   100%
pystac_client/_utils.py                 11      0   100%
pystac_client/cli.py                   159     43    73%   46, 50-51, 59-71, 103, 105, 115-116, 118-119, 127, 129-130, 269-277, 292-293, 296-301, 335-342, 349-351
pystac_client/client.py                149     14    91%   45, 353, 356, 374-375, 405-408, 428, 456-459
pystac_client/collection_client.py      80     12    85%   27-29, 97, 101, 108, 145-148, 181, 193-194
pystac_client/conformance.py            29      3    90%   30, 35, 38
pystac_client/errors.py                  3      0   100%
pystac_client/exceptions.py             10      0   100%
pystac_client/item_search.py           367     22    94%   34, 46, 97-100, 128, 284, 293, 322, 367, 375, 396-398, 418, 425, 515, 544, 551, 566, 575, 613, 677, 750
pystac_client/mixins.py                 37      2    95%   19, 52
pystac_client/stac_api_io.py           114     10    91%   36-37, 210, 213-215, 220-221, 225, 280
pystac_client/version.py                 1      0   100%
pystac_client/warnings.py               29      4    86%   92-96
------------------------------------------------------------------
TOTAL                                  996    110    89%

[jsignell]

I'm starting to tackle these with the top priority being the cli