This patch converts all the call paths from 'Result' (which contains
just string errors) to 'ErrorResult', which holds
GanetiException-encoded errors. We can now return proper
OpPrereq/OpExec errors to the clients of the luxi/query socket.

The patch touches many files as we had to convert the entire call
chains in a single round. But it should be pretty straightforward
otherwise:

- change 'Result' into 'ErrorResult'
- add error annotations: change "Bad msg" into "Bad (XXXEror msg)"
- add a helper function for confd, where we don't send to client
  formatted exceptions, to convert back from ErrorResult into Result
- change tests similarly, where needed

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