Declare exceptions as final

In #300 and #310 I've missed out to set the new exeptions to final. Doing this now would be a breaking change and will therefore be done in v3.0.0.

- `\Redmine\Exception\ClientException`
- `\Redmine\Exception\InvalidApiNameException`
- `\Redmine\Exception\InvalidParameterException`
- `\Redmine\Exception\MissingParameterException`
- `\Redmine\Exception\SerializerException`