test: Separate out the exception handling

test: Separate out the exception handling

The tests currently catch a very broad Exception in each case. This is
thrown even in the event of a coding error.

We want to handle exceptions differently depending on their severity,
so that we can avoid hour-long delays waiting for a board that is
clearly broken.

As a first step, create some new exception types, separating out those
which are simply an unexpected result from executed a command, from
those which indicate some kind of hardware failure.

Signed-off-by: Simon Glass <sjg@chromium.org>