Simplify LUXI exceptions

Simplify LUXI exceptions

Having only one exception hierarchy makes catching them simpler. Before
ProtocolError would derive directly from Exception, but with this patch
it'll also be in the hierarchy defined by the ganeti.errors module.
Separating encoding and decoding errors is not necessary at this point
as they're never handled separately, and merging them removes a few
lines from the code.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>