python: provide more exception classes

python: provide more exception classes

To make the exception handling more effective in code using the
python bindings it is necessary to differentiate between the
different kind of failures.

Add an exception class for each status code and add a decode
classmethod to the NotmuchError class that acts as a factory.

Import the new classes in __init__.py so they can be easily
imported by anyone.

Patch modifed by Sebastian Spaeth.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>