base: Use repr() for printing exceptions

Exceptions print more clearly using repr() instead of str(), fix
in fetch and unpack tasks.

Drop part of the test which no longer makes sense after this change.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c9481625)
Signed-off-by: Steve Sakoman <steve@sakoman.com>