luxi: convert permission errors into exception

This patch adds handling of permission errors so that we don't show
tracebacks when a non-root user runs a gnt-* command. Since in the
future we'll have different permissions, we need to handle this in RAPI
too.

It also fixes a typo in RAPI error message and the docstrings of LUXI
errors.

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