Correct some multi-line conditional indentation by slight refactor
if necessary.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

The lint tool dislikes the use of `if len(SEQ) == 0` type statements
because nominally `len(SEQ)` may be more expensive than `bool(SEQ)`.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Python linters prefer that attributes are defined in __init__ rather
than left to later routines.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

The imports of `Message` and `MessageType` were unused.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

In order to quieten the linter, and to make a more correct comparison,
we switch to using the `Code.OK` enumeration from `google.rpc.Code`

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>