We need to explicitly cast these exceptions to a str before using them - as
we're concatenating the messages.

The ValueError is thrown by the JSON parsing, so the response line should
be moved outside of the try/catch statement, so that we can be sure that
the `response` value is actually defined for the catch statement.

There is no need to assign these client tests to the object itself, as the
result is tested within the method. The assignment _could_ have side
effects somewhere later (if the teardown for some reason isn't ran as
assumed).

To avoid any potential side effects we keep the assignments local to the
method.