Fix exception re-raising in Python Luxi clients

Commit e687ec01 (present in 2.5 since the 2.5 beta 3) did consistency
fixes across the code-base. Unfortunately this was done without enough
checks on the actual meaning of one of the fixes, which means error
re-raising in lib/errors.py is broken.

The problem is that:

  raise cls, args

is different than:

  raise cls(args)

And our unit-tests didn't catch this (this patch updates the tests).

This breakage is usually trivial, like wrong error messages:

  $ gnt-instance remove no-such-instance
  Failure: prerequisites not met for this operation:
  ("Instance 'no-such-instance' not known", 'unknown_entity')

versus:

  $ gnt-instance remove no-such-instance
  Failure: prerequisites not met for this operation:
  error type: unknown_entity, error details:
  Instance 'no-such-instance' not known

or:

  $ gnt-instance add … no-such-instance
  Failure: prerequisites not met for this operation:
  ('The given name (no-such-instance) does not resolve: Name or service not known', 'resolver_error')

versus:

  $ gnt-instance add … no-such-instance
  Failure: prerequisites not met for this operation:
  error type: resolver_error, error details:
  The given name (no-such-instance) does not resolve: Name or service not known

But in some cases where we rely on a certain data representation
(e.g. HooksAbort), this actually breaks because we try to iterate over
the wrong type:

  File "/usr/lib/python2.6/dist-packages/ganeti/cli.py", line 1907, in FormatError
     for node, script, out in err.args[0]:
  ValueError: need more than 1 value to unpack

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